Once you got profit then just stop it. Because if you change your profit to a percentage you could control your greed. 
it is difficult to think to stop the games once we got profit because our mind will tell us that we can win more money. if we can control our feeling to say it is my time to stop then we can stop right away. every time we win the game, it will make us become greed and if we cannot control our feeling then we can continue to chase more win which we cannot always win.