Go to medfriends.com and go to the USMLE calculator. Some people have put together a database that equates your score on several predictive exams (150 released, Qbank, MCAT, NBME forms) to the real thing. It will give you your 70% confidence interval. So for Miami's 78% on the released questions this website says his 70% confidence interval is 135 +/- 11.
I think it's interesting that the tightest confidence interval of the four values is the 150 released questions. The MCAT has a +/- of 17 points, and Qbank and the NBMEs have confidence intervals of +/- 13. I just hope our year's 150 released questions isn't abnormally easy compared to previous years.