Gradebook

How would you suggest handling the following situation with your gradebook?

We give several assignments each week, and the weekly total is 30 points.

We then want to drop the lowest weekly score. Or perhaps the lowest two weekly scores.

Then we want to determine the student's letter grade by looking at the total number of points (a whole number) rather than looking at a percentage (typically not a whole number). (this is after dropping one or two weeks)

(We prefer to use points rather than percentage because percentages require rounding, and keeping of scores to one or two decimal places. Many students will argue that a 79.8% should be a B because you can round it up that way. They will not argue if the number of points they get is under the B cutoff, even if it is very close.)

Anyway, I am new to Moodle, and I am struggling with using the gradebook for this sort of thing. It was all so simple just doing it in excel. If you can give me some help, I would appreciate it.

Thanks,
Tamara