[AleoMagus]

The actual calculations to determine a specific bankroll requirement or a specific ROR are:

B=-(SD^2/2W)LN(R)

r=EXP(-2WB/SD^2)

where,
W is your average profit per tourney ($)
SD is your standard deviation per tournament ($)
R is your desired risk of ruin
B is your bankroll ($)

These calculations assume that a player will continue to play at a certain level, and will not cash out profits. This is, of course, a foolish assumption. In reality, we will sometimes cash out profits, and we will sometimes move up or down in stakes.

Assuming we want a 1% ROR, and we have a SD of 1.7 buy-ins, this looks something like this:

ROI - Buy-ins required

5% - 133.1
10% - 66.5
15% - 44.4
20% - 33.3
25% - 26.6
30% - 22.2
35% - 19.0

Really, the old 30 buy-in rule comes from smaller buy-in players who can get 25%+ ROI. All the higher limit players then notoriously chime in that 30 is way too little. This is obviously just because at the limits they play they are far more likely to get 5-10% ROI and thus, require a lot more.

Keep in mind that if a player does decide to drop in stakes when they take a hit, they need much less. Also, as we are talking about a relatively small number of tourneys, very tilty players might need a lot more.

Oh, one more thing... if you do cash out profits, and keep a constant floating bankroll (say 30 buy-ins) then you will almost certainly go broke. In some way (even a small way), you should always keep your bankroll growing.

Regards
Brad S