I'm pretty sure on the practice exam they base the scores on the fact that someone in the "reference pool" got an absolute 100 (i.e. has all the questions correct). So if you divide the number you of correct responses over total number of questions, you'll get the "raw score" which will be adjusted up on the real exam since likely there will be very few people to get all answers right AND there will be people who will get very low score. I think this is how it works.
Ugh, I'm getting soo nervous.