In theory, the probability your true ROI is 0% or lower depends on a nonmathematical prior distribution. However, what you might ask instead is how likely it is that a break-even player would have an ROI as good or better than the ROI you have had. If this probability is low, then you can say that you have strong statistical evidence that you are a winning player.

As long as you have played more than a few dozen tournaments, you can use a normal approximation. I don't know whether people have made a spreadsheet for that yet, and it would not be hard to make one, but I'll tell you how to do the calculation.

Almost regardless of your playing style as a serious player, your standard deviation in a 9-player SNG is between 150% and 155% of a buy-in per tournament. In other forms of poker, you can make plays which break-even in $ which greatly reduce your variance, but despite common misconceptions, those rarely occur in SNGs. So, let us assume that your standard deviation is 155% per tournament.

After n tournaments, the standard deviation of your ROI is about 155%/squareroot(n).

Express your observed ROI in terms of standard deviations: Divide your observed ROI by 155%/sqrt(n). This tells you the number of standard deviations by which you are ahead of breaking even. You can enter that value into the first line of

this web calculator. The reported cumulative probability is the chance that a break-even player would have worse results than you have seen. 100% minus this is the chance that a break-even player would have better results.

For example, after 300 tournaments with an ROI of 15%, your standard deviation is about 155%/sqrt(300) = 8.95%. You are 15%/8.95% = 1.68 standard deviations ahead of breaking even. The chance that a break-even player would have results worse than that is about 0.95 = 95%, so there is about a 5% chance that a break-even player would have better results. That's close to the boundary of when people would start to say that you have strong statistical evidence that you are a winning player, but you don't yet have a good idea what your ROI is.