I guess no CBTs will give you the exact evaluation as powerprep, it's the closest one to the real test, and if you're scoring 500-550 on math, I guess you should focus on math and increase it to 650+, while reviewing vocabulary. don't forget 333barron most frequ.
so I would say, do PP several times, review math and practice from this forum, and review vocab, guess you'll be ok. just don't panic, it's not that difficult to get around 1100. good luck. :)