it is like a batting average, but is the average off all your class pi's. if you won every race in mwss you have a pi of 1000 in that class..if you finsh mid pack in mwss 500 pi, and these were the only two classes you ever ran in you have a overall pi of 750. 750 then gets multiplied by you gross points scored. thus a rider thad does a bunch of races may not get the class title if they finish poorly vs a rider that did fewer races but finished strong consistantly.
keep in mind that if you did one class 1 time on a whim and did poorly , say pi of 250, that class pi gets weighted in an equal amount as your other classes you may have high pi's in.. do another race in that class to try and bring it up.
These are the things you learn earning the coveted #2 overall plate 4 times in 3 years. 8)