Need Help Creating a Letter Grade excel formula to be used in SharePoint.

Hey everyone! Thank you so much in advance for your help!

Okay now the issue, I have read many different posts on the site that explain how to take Letter Grades Lets say for example A, B, C, D and F and add a numeric value to them and then average it and convert it back to a Letter grade which shows the average using another table to look it up - my issue is I'm using this formula for a SharePoint Survey and there is no way of looking up from another table to my knowledge

So my question remains - How do I Average out letter Grades and make the result a letter grade without having to add a separate table? Thanks!!