For single table games (STT) you can get away with less than 100 BIs in your bankroll. A less conservative but still completely fine system is to have 100 BIs for the next level, you are going to drop down to, if you lose. So for instance with 660$ you can play 10$ games, if the next lower limit is 5$. And then you stop playing 10$ games, if you drop below 500$. Its also important to remember, that you are always allowed to play lower, than your bankroll allowes, and mixing up limits will increase game selection and reduce waiting times. So for instance with 660$ maybe you play some 10$ games, but you also still play 5$ games or even some occational $2-3 games.