GMAT official Guide Math Supplement Grading?

You may read in a few places that the GMAT tries to discover the difficulty level at which you're answering half your questions right, and half wrong. That's not technically correct; the 50/50 level is used by similar tests which are 'free form', with no answer choices. Because the GMAT factors in the possibility of guessing correctly from the five answer choices provided, if you only answer half your questions correctly at a certain difficulty level, the test will assume you're below that level. You really need to be getting at least 60% right at a certain level to convince the test that you are at that level (and the percentage can actually be higher, depending on the nature of the questions, but these details aren't important to know much about).

As for your question, it's not possible to answer with any precision. No one knows, for example, the precise difficulty levels of the questions in the OG. That said, if you're answering 85% of the questions correctly, you're doing very well, and you'll get a good barometer of your current level by taking one of the GMATPrep tests.

You can download the official GMATPrep practice tests for free from mba.com (there's a link called something like 'tools to help you prepare'). You need to register there first, but you need to do that anyway to book a test. GMATPrep is by far the most reliable indicator of your level, and I wouldn't put much stock in the score you get from a company test - some are very good practice, but none use calibrated questions, the official scoring algorithm, etc. Some give reasonably reliable scores, though not as reliable as GMATPrep, while others are very inaccurate.

I'd add that the database underlying each GMATPrep test is large enough that you can do each test a few times without seeing many repeated questions.