Penultimate Bimester Exam

I spent yesterday (Saturday) putting together my next bimester exams. I get a little jaded as I try to fill up 2 hours of time with the facts, concepts, and some skills (making data tables and graphs) from the past 9 weeks. I read someplace that it takes 20 minutes to properly teach a concept and 35 to teach a skill. Also, I've read that kids in best of situations retain somewhere between 10-15% of what they are taught. (I also read that over 90% of statistics are made up.) Teachers out there will have a feel that these numbers are close to accurate, though.
I have about 1800 minutes of teaching time in 9 weeks. I need to take out a week for the exams and half a week for review. That leaves about 1520 minutes per week. At 20 minutes per concept that would mean I should have 76 concepts to test. My test are about 120 questions long with a few skills and calculation type problems. Eighth grade are  velocity and acceleration and balacning chemical equations. Seventh grade are calculating density and molecular weight. Sixth grade are figuring out metric conversions and hours of daylight. If they remember 10%, then they should hang on to maybe 5 concepts and one skill. For how long will they hang on to that 10%? I'll need to do soem research on that.
Earlier in the school year I wrote a blog on SuperMemo which is this computer program that helps you learn stuff. My rudimentary understanding is that it helps you figure the optimal spacing for your review of material that you have prioritized to learn. In December I tried an experiment with some obscure (to me) Spanish verbs. I made 3 lists with 10 verbs on each list. I then studied one list every night, one list every other night, and the final list every third night for about 3 weeks until I knew all three lists perectly - 100% every night in a row for a week when my wife quizzed me. Since Christmas vacation I haven't looked at the lists. Last week I asked my wife to quiz me again. I got 8 out of 10 on every list.
I don't review facts every third day with students and certainly not 30 facts at differential intervals over and over for 3 weeks. If I did how would I choose those facts or concepts? I suppose it works out to about the same as the 76 concepts I figure I'm testing on the bimester exam. Maybe I should make the bimester exam in advance and give the kids 20 questions every day, 20 questions every other day, and 20 questions every third day for 3 weeks. Then move on to the next 60 bits of chosen information. Without review (if any are close to my pattern of recall) they would score around 80% with no studying. Of course what is the point?
Anyway onlh one more of these wretched series of bimester exams to prepare.