Vigenere cipher, can encrypt - decryption is troublesome!

hi there, i'm currently working on a spreadsheet that can encrypt and decrypt plaintext, using the vigenere method.

I can encrypt the text successfully as you can see in the attached worksheet, however i'm having trouble with the decryption. I know the maths formula to work it out is the following, but I just can't figure this out in excel.

Ma = Ca – Kb (mod 26)
(Where C = Code, M = Message, K = Key, and where a = the ath character of the message bounded by the message, and b is the bth character of the Key bounded by the length of the key

I don't think i'm too far off, any help would be appreciated

Thanks! this is great, i've never actually used this user defined function before, makes it a lot less cluttered