[pzhon]

The standard deviation per SNG is about 1.7 buy-ins. In 300 tournaments, it is sqrt(300)*1.7 ~ 29.4 buy-ins.

You expect to be up by .15*300 = 45 buy-ins. Losing 11 buy-ins or worse is 55.5 buy-ins below average. That's 1.885 standard deviations below average, which happens about 2.97% of the time.

That's quite likely to be an underestimate if you are not completely sure what your win rate is. If, a priori, you felt there was a 20% chance that your win rate was 13%, 14%, 15%, 16%, or 17%, it would be much more likely to lose 11 buy-ins, and you would also have more evidence that your win rate is more likely to be 13% rather than 17%.