I think the best way to use the NBME test is this:
Average = 500
Standard deviation = 100
(Your score - 500)/100 = x (this is a z score)
In this formula, your score cannot equal 500 (which would be the average on the current exam, obviously).
Example:
NBME score is 690
(690 - 500)/100 = 1.9
The current Step 2CK exam has the follow stats (I think):
Average = 226 (?)
SD = 23
226 + (x)23 = predicted score on USMLE from NBME test
You can then determine what percentile you are in from your z score:
http://www.epatric.com/documentation/statistics/z-score_table.html
Like if x = 1.9,
Your predicted score would be 226 + (1.9)(23) = ~ 269
You would be in about the 97th percentile.
At least for the Step 1 score report, the SEM was +/- 6.
Therefore, the projected score could be assumed to be 269 (263 - 275) given the SEM. This latter range could be thought of as a confidence interval.
Some people claim their Step scores went up, but note that both the mean and standard deviation are different for Step 2CK. So calculation of a z score and usage of the following table will help you know what percentile you achieved!