Barely cracking 50% on one of three practice exams doesn't sound like ready to me. But I never used Peterson's practice exams, so take it for what it's worth. I think you should buy a copy of the CLEP Official Guide, and you should score at least 60% on the practice exam there before you schedule the real exam.

Oh and questions not answered do count against you -- they count as a wrong answer.

-Gary-