Excel Function for caesar cipher

Hi guys! I'm working on a business project on Excel and got lost on the last section /: I would really appreciate it if someone could help me with it and also explain how you were able to develop the formula (Im not very good with Excel yet)
These are the instructions:
The encryption can also be represented using modular arithmetic by first transforming the letters into numbers, according to the scheme, A = 0, B = 1,..., Z = 25.[1] Encryption of a letter x by a shift n can be described mathematically as, En(x)= (x+n) mod 26.
When reporting grades, it’s important to have an authentication code to ensure that the information is valid. For this task, you will encrypt the first four characters of your last name using a Caesar Cipher, where “n” (offset) is 10 plus the last digital of your SID. For example, if you SID is 12345678, you would use cipher 8 to encode your last name.
develop an equation using the Excel “VLOOKUP” function to encode the first character of your last name using the Caesar Ciper table included in your dataset based on the last digit of your SID. A correct function would only require one to change one value (the last digit of your SID) in the equation to use the function for another person.

I attached the workbook with the table for it and with the number I'm supposed to use and also what to translate.
I've tried doing this on my own but I just couldn't figure it out
Thank you