How do you convert a percentage to a percentile?

You can't convert that directly. To convert percentages - or any kind of numbers, for that matter - to percentile, you need to use the definition of percentile. For example, if you have a grade of 90%, you check how many other grades have less than 90%, and divide that by the total number of grades. Note that for example with a grade of 90%, your percentile can be anywhere between 0 and 100 - depending what grades OTHERS had.