You can't "convert" EK scores. I suggest that you look at the MCAT forum, there is a Google Doc there with practice exam scores and actual exam scores from hundreds of 2015 applicants. Anecdotally, I scored high 70s / low 80s on all of my EK exams, and scored 520 on the real thing.
I can tell you that most find the EK exams harder than the real thing. But the question style is still the most representative, in my opinion.