From personal experience its a great indicator of your score. I would agree with +/- 30 pts or so. I scored 740 on GMATPrep 1, with 7 repeat questions from my first try at it 2 months earlier with no study time. Then on the second GMATPrep, which I had not taken yet, I scored another 740. This was only 2 days before my test day, and I scored a 760. That's just me, but to me, the numbers are very close.