How do I figure SNG winning %

I have been keeping records on my SNG play lately and need some help crunching the numbers.

First, I am trying to find out if I do better with 5 and 6 seaters or 9 and 10 seaters. Do I take the total buyins divided by money won for each to come up with winning percentages?

Second, assuming I am right on the first, what is a good winning percentage for SNG's and how many should I play to get an accurate percentage?

(Total winnings) / (Total buyins) = ROI

For example, you play 20 $10+1 SnGs and win $250 total...

ROI = 250 / 220 = 1.136 = 13.6%

Thanks for the help Dorkus. The math seems backwards to me but I understand it I think. Anything over 1.00 is winning, anything under 1.00 is losing. So far I have kept track of my last 40 SNGs, mostly $5 and $10 buy ins, and I have a ROI of 1.717 or 71.7%. Is this a good percentage? I plan on playing about another 60 of these and then moving up to the $20 tables if my numbers stay about the same. As far as the 5 and 6 seaters vs. the 9 and 10 seaters, I think I will generally play the 9 and 10s because I seem to place in those just as often as the 5 and 6 seaters. And by the way, what does ROI stand for?

Yes. Don't expect to be able to sustain it though, even at the $5s.

I've never actually looked it up, but I always presumed it to stand for "Return On Investment".

Aye, it does stand for Return on Investment, a commonly-used term in finance...