Trouble with CODE/IF functions

Any help would be very much appreciated on this one. for a course i am doing i am required to make a spreadsheet tracking student grades, which i have completed apart from one snag.
I have used the CODE function to convert the grades to numerical values, e.g. A is 65, B is 66 etc. which has worked fine, except for the difference between F and U, which are only one grade apart yet 5 numbers apart.
This has caused problems in a function which i am using to show how many grades above or below their predicted grade a student is, as when a student is predicted a F but acheives a U they are shown to be 5 grades below their predicted grade. the formula i used to show this is =IF(CODE(F8)=CODE(E8),0,CODE(F8)-CODE(E8))
F8 being the predicted grade, E8 being actual grade.

I am trying to find a way which will show that a student is only one grade below their predicted if they are predicted a F but acheive a U. prefereably without using a LOOKUP

Thanks everyone

Unfortunately, seems to show -3 regardless of what grades are entered in either columns. trying to work out why as we speak

Examples:

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i will try the match function again but couldn't get it to work beforehand

Got it working thanks guys for your help