There are free templates available for your specific needs. I would recommend using Teacher's Grade Book Template for Microsoft Excel® - by Jon Wittwer
This free Gradebook Template for Excel started out as a very basic grade book spreadsheet, but it has evolved into something that is very useful, flexible, and powerful (and still free). We now have a few different types of grade books that can handle most of the major grading systems used in high school and colleges.
It has the following features:
There are 3 available templates to download.
In your case I would use:
Gradebook Template : Percentage System
Just copy the Gradebook spreadsheet and name it as GradebookWithLetters or something similar and then put the data there. Then you can use the following function to match the letter grades to numerical values.
I cannot give you the modified template or share screenshots as it is against the License Agreement however I can help you modify your own template to match your needs.
Just put this in C11 in Gradebook (not the one with letters) and drag through the whole table.
Then just replace the values in Points/Weighting with 10% = 10 and so on. (Do not use percentages there as it messes up the calculations)
The main page contains a lot of tips and explanations on how operate the template.
I've used this template to calculate grades for a family member of mine who is a teacher.
All the credit goes to the creator of the template.