Hi Tim,
Unfortunately, the Diagnostic Test at the beginning of the
OG is not a great way to get a realistic assessment of an Official GMAT score. Relative to the Official GMAT, that Test does not have the same number of questions, the same ratio of questions, it doesn't adapt, it's missing the Essay and IR sections, etc. While it can help to define some of the areas that you need to work on, if your goal is to have a realistic assessment of how you might score on the Official GMAT, then the only way to get that result is to take a FULL-LENGTH CAT under realistic conditions.
The most realistic CATs on the market are the 4 GMAC CATs (the 2 free ones and the additional 2 that can be purchased from
www.mba.com). While there are other CATs that are 'close-enough', the reality of that initial CAT is important - since that score result provides a baseline and defines your initial strengths and weaknesses. In that way, having the MOST accurate initial assessment is important. To that end, you should also plan to take the FULL CAT (including the Essay and IR sections), so that we can see how you handle the endurance/fatigue 'challenges' of the Full Exam.
Some Test Takers become overly concerned about 'wasting' one of the GMAC CATs on the first attempt, but there are 3 other GMAC CATs and a variety of 3rd party CATs that you can use during the course of your studies, so that concern seems more emotional than practical.
GMAT assassins aren't born, they're made,
Rich