Assigning a value to a letter

This is a straight forward spreadsheet each number (color column) has a wt(white column) that it is associated with. There is a half time score, which is the addition of values between column B and I and a total score which is the a combination of values from B to Q

But let's say in the White column I wanted to put the letter M and have it read M on the spreadsheet but keep a value of .5, how would I do that? Spreadsheet attached.