So here's the deal: 35% class presentations, 35% handouts for the presentations, 10% revision of those handouts, 12% attendance and class participation, and 8% for "peer review" (offering constructive criticism of other students' handouts).
I picked a number for each category for each student. For the presentations, the basic 35% was reduced if you did less than about 100 minutes total of presenting. For attendance and participation, I took attendance about a dozen times. I made notes when there were particularly helpful questions and suggestions from the audience.
The result of this numerology was a number between 0 and 100 for each student. I picked grade divisions based in part on clustering of the numbers, and in part on my overall impressions. The A's were above 75, the B's were around 65, and the C's around 50.
Have a great summer!