I think what I want is really simple and common but I cant find a way to do it in moodle's gradebook. I have a grade category named for example "assignments". My course's total grade will be from 20 and "assignments" maximum grade is from 16. I want to add different grades with different grades to this category and at the end it calculates sum of the grades and converts it to a grade from 16. for example:
so if we have G1=90 and G2=8 and G3=18 then their total will be 116 and the assignments category's grade should be 14.27 ( = (90+8+18)*16 / 130 )
many grades may be added to the category during semester so its not an option to change the formula each time.