Hi rocko911,
GMAC has publicly stated that the Official Score that you earn on Test Day is within +/- 30 points of actual ability. Your 5 most recent CAT Scores show a bit of a larger 'swing' though, but they are within range of one another (about 550 +/- a few points). Since the Official GMAC CATs are the most realistic - and it appears that this last CAT was the only Official one in the last 5, there's some question as to your actual 'ability level' right now (is it closer to 500 or 600?). Until we can get a better sense of the answer to that question, it's difficult to say when you should schedule your Official GMAT.
Right now though, you should do a full review of this recent CAT, with the goal of trying to define WHY you got questions wrong.
After reviewing each section, how many questions did you get wrong....
1) Because of a silly/little mistake?
2) Because there was some math/verbal that you just could not remember how to do?
3) Because the question was too hard?
4) Because you were low on time and had to guess?
5) How many Verbal questions did you 'narrow down to 2 choices' but still get wrong?
GMAT assassins aren't born, they're made,
Rich