Our numeric values for our letter grades at our school are the following:
A+=97
A=91
A-=86
B+=83
B=80
B-=76
C+=73
C=70
C-=66
D=62
F=50
We have since found out that if you do this and give someone an A in a letter graded assignment it will calculate that as a 93.81 instead of a 91 (91/97). Why wouldn't it just give them the numeric equivalent?! To get around this we had to introduce a grade called '100' with a value of 100 so that it would calculate properly.
I completely agree! Although we don't use letters in our gradebooks, there are other areas in the system that do this to the grades too. It pretty much makes the Averages by Department report under Academics>Analyze, and other analyze functions that look at grades useless for us.