Set the letter's value in the Refers To box. You can create a value from data in a cell in the worksheet or you can add your own number. By default, Excel puts a reference to the currently selected cell in the box. If this contains your value, keep the default.
Gravely (the adverb) comes from the adjective, which comes from Latin (gravis), where it meant heavy or serious (this is where we get gravity--heaviness). Gravely ill means seriously ill, not soon to be in the grave. (and proof [1]). Bonus tidbit: grave (the noun) comes from words that originally meant to dig then a place dug, as for burial or, a grave. This is the same root as engrave.
I think I remember reading that injured means bruises and lascerations, seriously injured meant hospital but not life threatening, critically injured meant hospital ICU time and fatally injured meant injuries leading to dying sometime later.