Incentive

It's been a week I struggled with incentive calculation.. But until know i still don't find the right formula.. My friends already taught and explained the formula to me.. But I think it's not enough.. I need an example and simulation.. And when I try to create the simulation based on the formula that my friends gave me, it still doesn't work.. the result from manual calculation and from database is always different.. arrgghh.. and i don't what to do anymore.. i'm stuck with this.. and i'm very confuse.. why is it so difficult to calculate incentive?

BTW, can't wait until tomorrow.. cause tomorrow i will go to the doctor.. get facial and acne injection.. I think I already addicted to that doctor.. hehe.. get well soon my skin..