Yeah, I never figured it out, for either the diagnostic or the simulated exams. I actually emailed Kaplan and they explicitly said that they don't give out score correlations. Sucks, but w/e, it's still useful to sit for 7 hours straight to know what the real exam feels like.
That said, this might be reasonably predictive
http://usmle-score-correlation.blogspot.com/
Although I think it's meant more for the end QBank percentage, not the diagnostic exams