I took Knewton's course and worried about the scoring as well (I was getting 600-620 on ALL tests - including the last one). All you can do is your best - do the tests and the homework. Rather than counting the problems you got wrong, just review them. It doesn't do you any good to count them as even the highest scorers are going to get a number of them wrong.
Review and work at what you get wrong and by the time you're finished Knewton's program you should be getting a score that is more representative of your ability and less representative of Knewton's algorithm (which I think is a lot less helpful than their actual course) - although you're going to be using a different CAT source. I went from 610's-620's on my Knewton tests to 660/670 on my GMAT Prep tests almost immediately after finishing Knewton and finally a 660 on the actual test. So there wasn't too much in common with my Knewton CAT scores.
"You may not realize it when it happens, but a kick in the teeth may be the best thing in the world for you." - Walt Disney