After reading Ed Miller's book on NLHE, I like and use this approach, and unless the table has at least two deep unknowns or known bad players, I have had good results buying in for 60bbs. The typical table for me at 100NL is 1-2 sethunters 100bbs deep, 1-2 good players 100bbs deep, and the rest short, mostly bad players. 60bbs is enough to cover the bad players, and if I happen to run into the deep stacks, I immediately save 40bbs.