The 300BB bankroll comes from a combination of Kelly betting theory and risk of ruin calculations. Here are a couple of links that are interesting:
http://www.bjmath.com/bjmath/refer/RevGROR.htm
http://www.bjmath.com/bjmath/refer/CalcGROR.htm
http://www.bjmath.com/main.htm
I would use a 300BB bankroll and move down at a minimum when your bankroll drops to 200BB. Of course this all depends on your win rate. The higher the win rate the more flexibility you have with bankroll size. However, if you are not a winning player you will need and infinite bankroll.