[jason1990]

Well, "hopelessly inaccurate" may be an exaggeration. But the best you can say right now is that your SD is somewhere between 20 and 30. So, for example, suppose you have a 500BB bankroll and you assume your winrate is 2BB/100. Then your risk of ruin is e^{-2000/s^2}, where s is your SD. Using s = 20 gives 0.7% and using s = 30 gives 10.8%. So all you can say, in this particular case, is that your risk of ruin is somewhere between 0.7% and 10.8%. That's not a very precise range, but it's the best you can do without a more accurate estimate on your SD.

Or, conversely, suppose you assume your winrate is 2BB/100 and you want to know what size bankroll you need to have a risk of ruin of 1%. Well, you can't say for certain, because you don't know your SD. The bankroll you need is given by the formula

b = -(s^2/4)ln(.01)

where s is your SD. Using s = 20 gives b = 461. Using s = 30 gives b = 1036. So all you can say is that you need a bankroll of somewhere between 461BB and 1036BB. Again, not a very precise range, but, granted, it does tell you *something*.