Here on the right is my 5K RPI. Note that the first line of details is race-specific, in this case its with regards to the Women's running 5k. I finished in 29:42. Now, my RPI is 36.7, which means I should fare better than 64% of the 5Kers out there in my peer group. This is based on 4 5k races that have been logged into athlinks. The following two boxes (best and average) are based on the selected race shown.
The more races that are entered into the athlinks system, the better and more accurate the results. I think it will be interesting when they come out with some predictive measures, once they are able to compare elevation/temps/etc with the races you have run to the races that you are interested in running!