Sorry to revive a dead thread but this is the sort of question which gets asked a lot, I wanted to produce the table below but only just got around to it.
The confidence level is determined by two things only: your win % and the number of SnGs played. Of the two the latter - number played - makes by far the biggest difference (as you can see).
How to interpret the table? Well lets say you've played 250 HU SnGs at a given level and you've won 55% of them. Looking up the above table you should see this gives you the value 6.2%, this means that you can be 95% sure that your true win rate is in the range 55% +/- 6.2% (i.e. 48.8 to 61.2%). If you are playing with a 50+2.5 type structure, as stars has, you are trying to gain confidence that you are winning more than 52.5% (the break even point); this range dips below 52.5%, so you can't be sure you're a winning player.
Hope that makes it clear.
Cheers,
RH