Heres what I do. I decided a long time ago that I wanted to reach XX amount before spending a dime of it. Once I reached that point, I decided that at the end of each month, I would take out 75% of my winnings to spend and leave 25% in my roll for further bankroll growth.
Now I can play pretty much any game I want without worry about a bad streak killing my bankroll, and I still get to enjoy my hard work. I think its much better to wait until the end of each month instead of spending it each time you go above whatever threshold you use.