I typed up a response but decided not to post it because it isn't clear to me what exactly you are trying to get at. Which of the following are you trying to do?
(a) Given some tournament results, calculate an estimate of PPT and ROI and then put a confidence interval on it.
(b) Given some assumptions about x, y, and z, calculate the variance of PPT/ROI in order to get an understanding of how variable your winnings are.
(c) Something else.
I have played a lot of SNGs in my day, and have done all of these things at one point or another.
If you don't care at all about the underlying mathematics and just want to learn about your profitability, I know that there are spreadsheets available on 2+2 forums and elsewhere that are designed to handle (a) and other related things. I think the usual software packages (e.g. Poker Tracker, Holdem Manager) will do this as well, but maybe not confidence intervals. I don't think I've come across (b) publicly, although I'm sure it has been done many, many times.