[pzhon]

The main problem is that you estimated the probability that you would be down 20 BB at the end of the period, but we want the probability that you bust out at some point, which is significantly higher. It's a lot harder to estimate that.

There are some questions about using the normal/Brownian approximation. If you have enough hands for your end result to be roughly normal, that does not imply your downswings are roughly the same as for a Brownian motion. I'm not convinced that this makes a big difference, but with only on the order of 100 hands, the normal approximation itself is suspect.