It's next to impossible to estimate what you'll get by looking at your percentage correct on OG questions and nothing else - even when you limit yourself just to the hardest questions. Timing is a major factor, for one thing, and not just per-question timing but management of time over the entire section. Stamina is another major factor.
At a 750 level, you'd expect to get maybe 70-85% of the questions right (depending on the actual subscores you're getting in each section), and those questions would range from pretty hard to extremely hard (with the possible exception of experimentals, but of course those don't factor into your score).
It's much better to gauge your likely scoring level based on practice tests (especially GMATPrep).
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Stacey Koprince
GMAT Instructor
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Manhattan GMAT
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