The GMAT Prep tests tend to be the most accurate out there. There is a small margin of error (let's say +/- 50 points to be generous), but I'd be pretty confident that your "true" score at the moment is somewhere around 700.

As for the number of questions wrong - your score is based much more upon the difficulty of the questions you get right, not simply the raw number of right vs. wrong. The GMAT is adaptive, meaning that as you get questions right, the level of difficulty rises. Eventually, once you are into the upper bin material, you can get quite a few questions wrong and still score well. This has been discussed at length here on the forum - do a search if you want more details.

Hope this helps!

I am not sure how reliable that is either. I did the same thing as you (took it w/o much prep) and score 760! On the other hand, all OTHER prep test that I have taken (mostly Kaplan) were in the high 600s-low 700s area. Not to mention that on the 760 test I had 6 wrong answers in quant, which seems too many

HOWEVER, Jim knows much more than I do so I am gonna trust his answer.