I can understand; as gamblers, we get tempted to place bets. The best way to counter this is to set a profit limit or target. Once the target is achieved, make the withdrawal. If you keep your balance in the casino, then you will eventually get the temptation to place a bet and will continuously want to make more money. Hence the best thing is to just withdraw the funds after a certain amount is made in profits. Withdraw the initial amount you used, and if you really want, then gamble with the previous game’s profits.
I don't think it is proper to have an amount that you will win before you can withdraw. As long as the funds can be withdrawn, just withdraw it. This is because if you want your wins to pile up to a certain amount before withdrawing, what if it does not get to that amount before losing all that you won back to the casino.
I have this instinct since I started gambling responsible that if it is not the money that I deposited into my gambling account, any other money there is not mine until I have made withdrawal, and since I started withdrawing without leaving any apart from the one I will gamble with latter, I have not fallen for the temptation of losing back my wins.