I tell them my age at the start and at the end. Actual age at the start. Subtract 1 year for every thrust. Subtract 5 years for every ml of cum. Then tell them the result at the end.
My best, so far, is -75.
I think you need to revise your calculations.
The amount of jizz produced by the average male is between 1.5 and 5 ml, so you're making about 70 thrusts per shag?