How to predict scaled MAT scores from practice tests?

I bought a book from kaplan. The practice exams have 120 questions. How do I convert the percentage I get right into an approximate scaled score? I know that 20 questions are supposed to be experimental (which is why I want the percentage) and I know that diagnostic exams aren't normalized like the real exam, blah, blah blah. So please don't tell me that it is impossible to predict. For example about what percent do I need to get right in order to get a 450? a 425? a 475? If you know the percentiles, that would be helpful too. Thanks!