You have the right ides, but you can't just add up the schools' rates and divide by number of schools. Barry and DMU only have about 40-50 first time board exam takers per year. I think that Temple has a bit more than that number, I know Scholl has a lot more, and AZ early classes have a lot less (for now, but their classes will grow fast now that they're accredited).
NY has a lot more students than any school except perhaps Chicago, and OCPM has also began accepting far more students than they did in the past. I think Cali keeps their sizes reasonable at about the average 40-50. As you see, if a school that accepts many students such as NY, OCPM, or Scholl had a poor pass rate, that'd affect the national rate much more than say, AZ's relatively small population of test takers having a bad year on the boards.
These are my just rough guesses, but here's the approx number of potential '09 grads at each:
Cali 40-50
DMU 40-50
Temple 50-60
OCPM 60-70
NYCPM 60-80
Scholl 70-80
AZ 30-40
Barry 40-50 (I think it's 42 or 43 for my class, but don't quote me on that... we just lost 1 or 2 yesterday)