How do I calculate a 100 point GPA to a 4.0 GPA
If your school also gives letter grades (A,B,C, etc.) then use your letter grades.
Otherwise, you can’t really convert 100-point scales to the 4.0 GPA scale. In many HS courses, a grade of A is anything >= 90, a B is anything >= 80 but < 90, etc. but it’s not exactly universal.