I once had a teacher who worked at a standardized testing center and she said that you didn't even need to fill it in half way. Just a dark mark in the center was sufficient. Though, with something as important as the MCAT I don't know if I would take that sort of a risk. Who knows, even if you get it graded by hand, you might get some anal person who says that your answer is wrong because you didn't fill it in all the way. 🙄