To complement all the good info above:
If you want to know percentile rank for a normal distribution (we have to assume the USMLE scores are close to normal... which is probably true), calculate a Z score.
Z = ((Your Score)-(Mean Score))/(Standard Deviation)
You can then look up the correlation between Z score and percentile in Z score tables like this one:
http://www.epatric.com/documentation/st ... table.html
For example, If your score was 248, the mean was 215 and the standard deviation was 15, then:
Z=(248-215)/15 or Z=2.2
Based on the table, a Z of 2.2 corresponds to a percentile rank of 98.6%
And, as stated by those above, a Z score of 1 (1 standard deviation above the mean) corresponds to 84%, and a Z of 2 corresponds to 98%
Again, this assumes a normal distribution, but it should be close for the USMLE.
By the way, does anyone know to what the 2 digit score corresponds? I know that at least a few programs pay the most attention to this score (or so I've heard from a couple P.D.'s). Near as I could figure when I took the test, any one with a Z > ~ 1.5 got a 99, but that is based on a few conversations with a few people.