Assign Values To The Grades And Insert Extra Columns To Do The Calculations

I have a list of grades they are 3a, 3b, 3c, 4a, 4b, 4c, etc up to 7a. An a is one grade than a b, and a b is one grade higher than a c. In one test a student may score a 4b and in another a 4a. I need a way to calculate the difference in "grades" e.g if a student scores a 4a in test 1 and a 5c in test 2 the difference is 1.

Is there a way I can do this using custom formatting or will I have to assign values to the grades and insert extra columns to do the calculations. I need the simpliest way as I have to disseminate to my staff in my department.