The 15% cash back is calculated using the following formula: (new deposits for the period) - (redemptions for the period) - (current & pending balance) - (winnings from previous periods) X .15 = (cash back award)
($500 new deposits for the period) - ($0 redemptions for the period) - ($0 current & pending balance) - ($0 winnings from previous periods) X .15 = ($75 cash back award)
so you are correct sir, If you start the season with $500 and end with $0 you would get 15% of the $500, not 15% of the total of all your loosing bets.
I can't see anyone giving you cash back on just your losses without taking into account your winning bets