Grade book with varied weightings; letters to numbers to letters

Hi everyone, I am looking to edit a markbook/gradebook. I am trying to change the weighting of each section. With the attached document, I am trying to make it so that SU = 40%, SHE = 40% and SIS = 20%.
So let's say they got an A+, B- and a C the overall result should be a B+. With the gradebook I need to make sure that it doesn't count N's or blank spaces. Thank you.