Money Management

Am I reading this right? I picked up a BJ book and the chapter on Money Management says don't walk up to the $5 tables with less than $120 per session, play three sessions and walk away from each when you win 20% or lose 40%. First off, those numbers seem very conservative to me. Secondly, why not just buy $50 in chips and walk away when you're tapped out or win $25? Thirdly, what's the difference between two sessions five minutes apart or five days apart?

Anybody out there use a different money management scheme?

Thanx in advance

TruFaux

P.S. Thank all of you for your thorough and sincere responses to my last post re: Players Cards.