## simple question considering percentiles

Dear Community

I have a question regarding percentiles. I am not sure if this is the right forum for this.
I want to create a game where one can win points. The amount of points won during each trial depends on the reaction time of the participant. Before the main experiment, a practice session with 20 trials is presented. For each trial a maximum amount of points is defined and a reaction time window during which the participant has to react in order to win points. The bottom of the time window corresponds to the 80th percentil, the top to the 50th percentile of the subjects individual reaction times. The percentiles are calculated for each subject individually, using his performance during the practice session (but also using an adaptive algorithm during the main experiment). If the participant reacts as fast as his own 80th percentile he gets the minimum amount (0 points), if he reacts as fast as his own 50th percentile, he gets the maximum amount (100 points).

I hope this is somehow understandable. My question is how can I calculate in advance the mean points won by each subject? I need to know how much a subject will typically win when I use this time window.

Thanks a lot!
Durden