Google Gradebook App - Extra Credit and Missing grades, auto drop lowest grade

Info and how my Google Sheets gradebook app is set up

it is weighted categories, Minor grades (homework/daily work/quizzes) is 40% Major Grades (Tests/Labs) is 60%

My request for help is a three parter

1st part. I would like to know how I can add extra credit. I do not give extra credit very often but would like to be able to add it. On extra credit I typically do not give more than 20 points (these are usually a separate piece of paper from their regular homework or test). I do not want to increase their overall average by 20 points but I would like to increase it by 3% (Total extra credit points for the 6 weeks is 50 to get the full 3% then all 50 points would have to be earned. If they got less than 50 points then a smaller % would be).

2nd part. Would it be possible on the mark column to be able to enter M for missing it highlight that cell red and be averaged as a zero until a grade is entered in place of the M?

3rd part. Would it be possible to drop the lowest grade automatically (or 2 of the lowest grades) from one or multiple categories?

Thanks in advance for your help

Johna🙂