quit while (one) is ahead

To cease, end, or give up (doing) something in which one has found some amount of success, especially so as not to risk spoiling or reversing that success. Look, that's good enough for what we need, so let's just quit while we're ahead. If we monkey around with the settings too much, we could screw the whole thing up. As soon as I made a bit of a profit playing poker, I decided to quit while I was ahead—no sense ending the night worse off than I began. I appreciate your compliment, but you should quit while you're ahead, because you're starting to veer off into creepy territory.

quit while one's ahead

Don't try to improve on something that is already accomplished, as in Those drapes we hung are even enough-let's quit while we're ahead. This idiom also implies that further action runs the risk of spoiling something. Also see leave well enough alone.