Don't forget that depending on the skill edge you have over your opponents, your bankroll requirements will change. For example if you are playing $.05/$.10 No Limit, you may be able to get by with only 15 buy-ins, because your winrate is higher, so you will have fewer and shorter downswings. At a higher game where your edge is tiny, you might need 40, 50, or even more buy-ins in order to play comfortably. Also remember that 6-max games have more variance than full ring games, so you need a bigger bankroll if you are playing 6-max.