How To Add And Subtract Using Letter And Numbers

I am looking to be able to compare a number with a letter so for example 4c and compare it with another 5c and in that scenario see a difference of 1 but when the two to be compared are 4c and 4a it would see 0.6.

in this excersize

A=0.33333R
B=0.33333R
C=0.33333R

it is basically sub category of the initial number so an A is better than a B and in tern a B is better than a C

for example

Targeted Working At Residual
4c 4b 0.33333r
5a 4c -0.33333r
4c 5c 1
4c 5a 0.33333r
5c 3b -1.66666

so on and such forth the number and letter can be entered into seperate fields the lettering are only A,B,C but numbers can range from 1-9 i am normally quite ept with excel but this is out of my depth