[Double Down]

Yes, it's impossible to know your true ROI, but as you said, you will tend to drift towards a number the more SNGs you play, and by using standard deviation, you can find out the chances of your actual ROI being within certain ranges.

For example, after 10,000 SNGs, if your ROI is 15%, there is probably a 99% chance that your ROI is between 13% and 17%. And after 20,000, there's probably a 99% chance that it's truly between 14.5% and 15.5%. I'm pulling these figures out of my ass, but someone who knows how to utilize standard deviation could figure out the actual numbers.