good sample size for ROI calculation

[Absalon]

Hi Paul,

interesting Post, but can you tell us where the 180% come from?
It seems to me, your Formula doesn't agree with 2+2 spreadsheet:
I have measured a ROI of +20.1% over 428 Tourneys. So by your Formula my SD should be 9% and I could be sure to be a winnign player at the 95% confidence Level, but not at the 99%, am I right?
But the spreadsheet tells me, my "Winning Confidence" is 99.6% and I can have a 99% confidence that my $/tourney is between 0.03$ and 2.17$ (5+05$ Tourneys).
So, according to you my winning Confidence is between 95% and 99%, but according to the spreadsheet it is over 99%. Another Formula? And why, where does it come from?