another factor is to check the weight of each paper. For example, I scored 79% in a paper worth 50%, 100% in a paper worth 30%, but around 55% in a paper worth 20% and ended up with an A overall (I was around 12-14 marks off an A*). Check the spec that you're with and familiarise yourself with the marking schemes so that you know any weaknesses that may cost you your grade. For me in my MFL writing paper worth 20%, I had one serious grammar error (misgendering a noun) which capped my A04 to maximum 12/20 marks on each of my essays. If I had the time to check my work when I finished writing, I probably would've got the grade. It can also help you to organise your revision (just ensure that you don't abandon the smallest contributing elements that make up your grade).
also if I'm not mistaken, one of my teachers remarked that grade boundaries are not formally set until all the marks are in. If 90% of students end up getting 90% on a paper then all of a sudden making 90% the requirement for an A* does not create a good look on the outcome of the exam. There will most likely be a certain number who pass and then divided into percentiles which form the boundaries for each grade. That isn't really something within your control, so there isn't really anything you can do except ensure that you do everything you can revision-wise to ensure you're in that highest percentile to give yourself the best fighting chance of getting that A*.