Once you got profit then just stop it. Because if you change your profit to a percentage you could control your greed. 
What if you don't get profit then will continue until you get a profit in gambling? Don't ever plan to gamble until you get profit but also try to set a limit for your losses. If you lose a certain amount then should stop gambling and come back again some other time. Setting limit only for profit may lead to losing a lot of money in gambling don't do that mistake.
That is too risky on my opinion, because the house won't give you the profits that you are looking for that easy, gambling is all about losing most of the time, I hope you know that. Most of the time you are going to lose more money first before you have this "profits" that you are looking for, and sometimes, you couldn't even make one, and your greed will grow even more because you seek the big amount of money that you wanted to win.
You have this mindset that you won't stop until you get some profits, but trust me, it is not worth it to try. Getting more greedy will result to an outcome which you might want to happen.