"Beyond feel and experience"? Using a mathematical formula to create a ritard in music would not be beyond using instinct, but vice versa. It's nearly impossible to create an authentic ritard in computerized music because composers don't create ritards to slow down the music, but to create an emotional reaction in the audience. The rates of ritards depend on countless criteriae such as harmony, melody, timbre, and rhythm, and what the musician has had for lunch.

Computers are thus extremely impractical music makers. It is up to you to decide what is pleasing and program the computer to follow your wishes, if possible. Whatever you decide is pleasing, it is extremely probably that someone out there will disagree with you. The best you can do is make an intelligent decision about the music.

In terms of following ritadando markings in the score:

At the end of a piece

Ritardando markings at the end of a piece should generally be greater than other ritardandos throughout the piece, unless otherwise specified. If the music was slow to begin with, musicians should be careful not to slow too dramatically.

When there is a dashed line (- - -) indicating the length of the ritard

Musicians should gradually slow the music down until the dashed line ends in the score. There may or may not be a tempo indicated to arrive at. Obey any "a tempo" markings.

When there is a ritard marking alone

Obey the ritard marking intelligently.

Mathematics of Ritardando
I'm working on some studio lighting and want to have my two Canon Speedlite 580EX IIs fire simultaneously using Canon's built-in wireless communications (i.e., I want the master to trigger the slave, but also provide some fill flash).  I am pretty sure I had this working at some point, but now I have noticed that the master flash fires when the shutter is pressed, but the slave flash fires a good 1 or 2 seconds after the master has fired (and after the shutter has closed and photo has already been taken).  I have been poring over documentation and videos wondering what I could have missed. Surely it is a setting somewhere that I have accidentally set?

MASTER 580EXII
Set to ETTL
Channel 1
On
Ratio A:B  is 1:1

SLAVE 580EXII
Channel 1
On
Group B

Notes: I have tried a couple of things:  


Connecting the master and slave with the mini phone jack. 
Making sure the master is directly in front of the slave's panel so there could be no interference with the signal. 
I have tried it with the Master ON and the Master OFF.
I have tried it with the Master both on the camera's hotshoe and on a jotshoe adapter. 
Even tried replacing all the batteries in both flashes.  
Have tried both flashes in Group A.


I will try to attach pics of the LCD panels.  I must be missing something obvious... please forgive me if I did.  Thank you in advance!

P.S.  I have the Master set for a -1 FEC but I have tried removing that as well.
