Unfortunately, your accuracy rate on the OG questions can't reasonably be translated into an actual GMAT score. For starters, the difficulty of the OG questions varies quite a bit... so getting 60-70% correct on the hard questions is much more impressive than getting 60-70% correct at the beginning of the book.

In any event, your GMAT score will ultimately determined by which questions you get right, not just by how many you get right. Even on the real test, the percent of questions that you get correct doesn't necessarily say much about your score. If, for example, you get 22/37 questions right on the quant, you could end up with an amazing score (45+)... or an absolutely tragic one. (CAT tests are pretty strange animals if you're not used to them.)

If you want to get an accurate gauge of your actual progress on the GMAT, take a GMATPrep test from MBA.com. That will test your skills under time pressure, and give you a much better sense of what test day will feel like.

Good luck!

The consensus is that the real test is harder than the OG questions, so attempt to improve your accuracy in the OG to around 80% or higher so you will be better prepared for the real thing when you see harder questions.

Practicing in the OG alone is not sufficient. The OG offers practice but no strategies, take-aways, guessing techniques, etc... For this you need another resource such as the OG Companion (from GMATFix, for the quant). For verbal, I think most people would agree that Manhattan GMAT has great Sentence Correction strategy guides and I've heard a lot of people praise the CR Bible (I haven't used it).

The GMAT is harder than the OG so eventually I think you should move beyond the OG and try to master the Qs in GMATPrep

-Patrick