referring now in detail to the drawings , fig1 provides a plan view of calculating device 10 . the calculating device , in its preferred embodiment , comprises a generally rectangular housing having opposing first and second sides 12 , 18 . both the first side 12 and the second side 18 are mated with a first edge 36 and second edge 38 opposing each other and extending generally parallel to each other while a third edge 40 and a fourth edge 42 oppose each other and extend generally parallel to each other but perpendicular to the first edge 36 and the second edge 38 . the device 10 includes an electrically operated control unit ( not shown ) positioned within the housing . the control unit includes calculating or processor means , as described in more detail below . the calculating device 10 is most preferably battery powered thereby allowing the invention to be portable and convenient to use . referring to fig3 the dimensions of the device 10 of the preferred embodiment allow the calculating device 10 to fit within a compartment for use within a folder 28 as is often used in restaurants for presenting meal charges to dining patrons . the calculating device 10 is preferably about the size of a standard credit card . preferably , the distance between the third edge 40 and the fourth edge 42 is three to four inches , the distance between the first edge 36 and the second edge 38 is two to three inches and the distance between the first side 12 and the second side 18 is 0 . 25 inch to 0 . 5 inch . the first side 12 , as seen in fig1 includes means for controlling the operation of the calculating device 10 . the first side 12 contains first 2 , second 4 , and third 6 calculating device control means . the calculating device control means 2 , 4 , 6 may be flat surface or raised surface push buttons , heat sensitive surfaces , electrical conduction based devices , data entry levers or knobs or other forms of data entry devices or control devices as commonly known by those with skill in the art . the preferred embodiment comprises flat surface or raised surface push buttons . the first calculating device control means 2 preferably comprises a button for energizing the calculating device 10 . the first calculating device control means 2 is preferably labeled &# 34 ; on &# 34 ; but could be assigned various labels as appropriate and known by those with skill in the art . the second calculating device control means 4 preferably comprises a button for de - energizing the calculating device 10 . the second calculating device control means is preferably labeled &# 34 ; off &# 34 ; but could be assigned various labels as appropriate and known by those with skill in the art . the third calculating device control means 6 preferably comprises a button which when actuated removes any residual values in the processor circuitry . the third calculating device control means 6 is preferably labeled &# 34 ; clear &# 34 ; but could be assigned various labels as appropriate and known by those with skill in the art . numeric data entry means 8 may be flat surface or raised surface push buttons , heat sensitive surfaces , electrical conduction based devices , data entry levers or knobs or other forms of data entry devices or control devices as commonly known by those with skill in the art . the preferred embodiment comprises flat surface or raised surface push buttons wherein the button , when depressed , causes an electrical connection to be made thereby signaling the processor circuitry . the numeric data entry means 8 preferably comprises ten buttons , each button preferably being sequentially labeled with one of the numbers zero through nine . the numeric data entry means 8 provides means for an operator to enter numeric values into the processing circuitry . as described above , the processor circuitry is sized and arranged to fit within the housing of calculating device 10 , as will be understood by those of skill in the art . also located on the first side 12 is a first side amount display screen 14 . the first side amount display screen 14 may consist of many types of display devices as known to those with skill in the art . preferably , the display screens comprise liquid crystal display ( lcd ) screens or light emitting diode ( led ) screens . all display means in the present invention are controlled and powered by the control unit or processor circuitry inside the calculating device 10 . all display means in the present invention are preferably sized to create a large display thereby making it easy for the operator to view the display . all display means in the present invention are preferably back or side lit thereby making it easy for the operator to view the amount display screen in dimly lit conditions . the first side 12 preferably contains support means 16 to elevate the first side a sufficient distance to prevent the first side control means 2 , 4 , 6 , and the numeric data entry means 8 from contacting a surface supporting the calculating device 10 . possible support means include legs , half spheres , ridges , protruding shoulders or other such protruding structures . preferably , the support means comprise a raised ledge 16 skirting the first 36 , second 38 , third 40 , and fourth 42 edges of the first surface 12 of the calculating device 10 . the ledge 16 prevents supporting structure such as a table or folder 28 ( fig3 ) from disadvantageously altering the amount input into the device 10 . fig2 provides a plan view of the second side 18 of the calculating device 10 . the second side 18 contains fifth 27 , sixth 29 , seventh 30 , and eighth 32 calculating device control means for controlling the operation of the calculating device 10 . the second side 18 also contains additional display screens for providing output from the processor circuitry to the operator . a second side amount display screen 20 is included on the second side 18 and displays the same amount as entered on the first side 12 . the fifth 27 and sixth 29 control means preferably comprise buttons as found on the first side 12 . the fifth 27 and sixth 29 control means are preferably assigned predetermined numeric percentage values . the fifth 27 and sixth 28 control means , when actuated , signal the processor circuitry to calculate the chosen predetermined percentage of the numeric value entered by the user on the first side 12 and displayed on the first side amount display screen 14 and the second side amount display screen 20 . in the invention &# 39 ; s preferred embodiment the preprogrammed percentage amounts are fifteen ( 15 ) percent and twenty ( 20 ) percent , those being the most common percentage values to leave for the service provider as a gratuity . however , other percentage amounts may be used , as will be easily understood by those skilled in the art . also located on the second side 18 is a seventh 30 and eighth 32 control means which allow the user to alter the percentage value in integer increments . preferably , the seventh control means 30 is labeled less and the eighth control means 32 is labeled more although various labels could be applied as known by those with skill in the art . the user may actuate the eighth selection means 32 or the seventh selection means 30 to increase or decrease , respectively , the percentage value in one integer increments . a percentage value display screen 22 displays the percentage value that the processor circuitry will use in the computation . a tip amount display screen 24 displays the percent amount , calculated by the processor circuitry , based on the operator entered data . this value will vary as the operator alters the percentage amount using the eighth control means 32 , labeled more , the seventh control means 30 , labeled less , or the preprogrammed percentage control means 27 , 29 . the processor circuitry sums the values displayed in the second side amount display screen 20 and the tip amount display screen 24 and displays the sum in the total display screen 26 . a method of using the calculating device 10 is as follows . an operator , such as a waiter , cashier or the like , turns on the device by pressing the &# 34 ; on &# 34 ; control means or button 2 . the displays and memory of the device 10 are cleared by pressing the clear control means or button 6 . next , the operator enters a data amount into the memory , which amount is displayed on the displays 14 , 20 . the operator enters the data using the data entry means 8 . this amount entered may correspond to the cost of a meal or other goods or services or the like . the device 10 is then provided to the customer . this device 10 may advantageously be provided in a folder 28 ( as illustrated in fig3 ) or on a tray or the like . the device 10 is provided with the second side 18 facing up . as noted previously , the ledge 16 surrounding the first side 12 prevents the various buttons from contacting the folder , tray or other support surface . the customer then enters the desired tip amount and a total of the amount entered by the first party plus the tip . the customer enters the tip amount by pressing either of the preprogrammed percentage keys 27 , 29 . the customer may alter this tip amount by pressing the &# 34 ; more &# 34 ; or &# 34 ; less &# 34 ; keys 30 , 32 . the current tip amount is continuously displayed in the display 22 . once the customer has entered the desired tip amount , this tip amount is displayed in the tip amount display 24 , along with a total amount in a display 26 . the customer may then provide payment of the desired amount and return the device 10 to the operator . after use , the memory and displays of the device 10 may be cleared by pressing the &# 34 ; clear &# 34 ; button 6 and a new amount entered , or the device 10 may be turned off by depressing control means or button 4 . while the above - described device 10 has been described as arranged to calculate a percentage of an entered value , the device 10 may be arranged to calculate any number of values based on the entered value . of course , the foregoing description is that of preferred embodiments of the invention , and various changes and modifications may be made without departing from the spirit and scope of the invention , as defined by the appended claims .