If you're actually a winner at 2/4nl (and all the stakes below) and only have $50 for some reason, the 20 buyin rule for you is terrible. You shouldn't keep yourself stuck in .05/.10 for a huge number of hands waiting to build your roll to $500 for .10/.25, you're earning severely below what your hourly rate would be at higher stakes.
See
http://www.twoplustwo.com/magazine/i...kratz0707.html for a good treatment on aggressive bankroll management, which should be most applicable in a situation of a good player whose roll forces him several levels in stakes below his normal game.