There isn't really a "percentage" with AP testing. You don't have to meet a threshold. You just have to score in the top 5% (correct me if I'm wrong?) in the nation. It's graded on a bell curve, not a rigid plot. And don't try to find your bottom limit, man. Just shoot for the 100%. Just know that you've got some firm competition right here. :)
This makes Calculus BC one of the most generously graded AP exams, right behind AP Chinese Language and Culture, where more than 70% of students get a 5 (Wikipedia: "Many native Chinese speakers take the exam, leading to an overwhelming majority obtaining the maximum score."). I figure this sort of self selection might also explain the BC distribution, and the distributions of other (relatively) highly specialized or difficult tests, as well...in general, there seems to be an inverse correlation between number of test takers and average score. Compare, for example, with Calculus AB (top 25% get a 5), Physics B (15%), US History (10%).
You can get all these stats and others from the College Board, but Wikipedia conveniently displays the scores, and sometimes the number of students.