Are you missing difficult questions or easy questions? If most of the questions you missed are harder 700-800 level questions then you should be ok...
I took the manhattan exams online and I remember getting 15problems wrong in the verbal section once and still got a 39 raw score (89%). The only questions I missed that day were 700-800 level ones.. Don't believe GWD too much even if you miss less than 10. The score really depends on the average level of difficulty of your questions.