You probably did the 150 questions that the NBME sends to all people who register for Step 1. I have not come across any solid correlations btwn that score and Step 1 score. The only things I recall hearing are that those questions do show up on the real thing and that most people agree that those 150 are a little shorter and easier than Step 1 as a whole.
It seems most people report scores >75% on that one.
RJ