I don't know if there really is a right answer to how much you should be playing with. I always thought it all depends on how many tables you play and how much you wouldn't mind losing. I don't particularly believe there's a certain percentage that should tell you how much you should have on the table. For instance, if you play 50 NL and you play about 10 tables at once, you've got 500 in play. Would you mind losing 4 or 5 buy-ins in one session? If you would then maybe it would be more profitable to play more tables at a smaller limit? As long as you have at least 50 buy ins of whatever limit you're playing you are ok. If you fall below that 50 buy-ins I'd move down in limits til you have enough buy-ins to move up. I wouldn't worry as much about the percentage of your bankroll you have in play.