Usually GMATPrep is within 40 points of the final score (up or down).
How long have you been studyig? Which is easier for you quant or verbal?
Achdee is right - you don't have much time to invest, so focus on the area that is easiest to improve.
You are in the area of 650 - just keep reviewing your error log
and taking full length tests to build up your stamina. However if you are looking for a higher score, you may want to reschedule to mid-Sept and save yourself $200
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