Score the tests normally, then organize them in a list, from the lowest score to the highest score. Choose a score. You will need the equation L/N(100) = P in order to score based on percentiles. The variable "L" will stand for the number of scores below the chosen score and "N" will stand for the number of test scores. P will stand for the percentile, which is what you are trying to find.
For each of the scores, work out this equation. For example, if you want to find the percentile for your standard score of 90, then begin with that. Count how many scores on your list are above the 90. Let's say that there are five scores higher than 90. Then, L = 5.
Count the total number of scores. Let's say that there are 10 scores total. Then, N = 10. Next, divide L by N, which would be the same as dividing 5 by 10 and getting .5 as your answer. Multiply .5 by 100 to get 50. This is the answer to the problem and the percentile score. 50. Do this for all of the scores to determine all scores based on percentiles.