I have no idea about the percentiles, but fwiw my ds' counselor recommended taking the Math II subject test after pre-calc. There isn't anything further he'd be learning in Calculus that would help on the test. Not sure what grade he's in, but one reason to take it sooner rather than later is that if you leave subject tests until junior year... chances are your ds might also be trying to take SAT/ACT/AP tests etc in the same time frame, and that's just a ton of testing to deal with, so getting what you can out of the way early can be a really good idea
Best wishes,
polarbear
