The first time I had a GMAT CAT with Knewton (before I did any kind of preparation) I scored 600. I found Knewton's CAT scoring reasonable although the best indicator would be the official GMATPrep ones.
Other than the number of wrong ones (although you didnt make too many mistakes) one needs to consider the difficulty of the correct ones thats what gives you the points. If you feel you have the material covered and you understand all the GMAT strategies etc, then there is no much use for the Knewton prep. On the other hand, if you do not feel you have covered everything, the Knewton prep is the best I've seen. One thing though to keep in mind, is that the Knewton prep (and I guess any prep) will not get you a high GMAT score on its own. You also need to do as many questions as possible.
So to sum up, Knewton CATs I think are reasonable, the Knewton prep class is really good if you feel you dont know the material or the strategies very well but at the end of the day, its practice with as many questions as possible (>500) that will get you the high score on test day. Also time management is important (never spend too much time on a single question, its the recipe for disaster).
For your case, since you only have 10 days left, you cannot do the Knewton class you need more time, so the best use of your time would be to do practice questions (all hard OG ones and something like Kaplan 800) and for every mistake make sure you understand what went wrong and learn how to avoid it, and do the same for the ones you take too long to answer, these are as good as a wrong answer (maybe worse).
Good luck with your preparation and test.
nOObaBa
Math - 6 mistakes
Verbal - 7 mistakes {all RC

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Score 570. Weird. I'm anxious since.
Guys, any opinions of whether the scoring algorithm on Knewton's is just designed to scare and covert you into a paying customer for their prep class.
Help please. 10 days to go...