Sorry I can't answer that question as most of the diagnostic tests I'm aware of provide you with scores similiar to GMAT, so there's no translation needed. Which test did you take to get the 510?
I would definitely recommend a pattern of studying, figuring out your weaknesses, then taking practice exams to estimate progress and feedback on what to focus on next. The official site mba.com has free tests that you should be able to take quite a few times before seeing repeat questions that skews the results.
BTW, if you have not read this already, check out this post (esp tip on how to best estimate your GMAT score): gmat-scores-83890.html
Also, from your comments about below average skills in SC and RC, you should focus those 2 areas then. Otherwise, you will not get pass to the more difficult questions -- which is what you'll face from the adaptive nature of the test.
And yeah, if you have luxury of more time... postponing test sounds like the right choice. So don't give up; there are plenty of folks who struggled with verbals and later did great!!