This forum really helped me while I was preparing for the BIG day. Here is a brief look at my practice test scores.
Kaplan Diag = 580
Kaplan(mutiple practice sets) = scored between 620 - 710
GMATprep_1 = 680
GMATprep_2 = 710
REAL GMAT = 710
GMATprep is the BEST indicator of your REAL GMAT performance. So, use it to measure your readiness level.