Basically winrate is used for cash games. Your metric for tournaments is ROI (return of investment).
1) How to calculate ROI:
ROI=Net Winnings / Total Investment=
{(Total Winnings)-(Total Buyins incl Rake)} / (Total Buyins incl Rake)
Example:
So assume that you have played 100 SNGs with a buy in of $5+$0.5 and won a total of 700$.
Then: Total Buyins including Rake=100*(5+0.5)=$550
Total Winnings=$700
And your ROI= (700-550) / 550 =0.27=27%
2) Converting ROI to Winrate and Winrate to ROI
Winrate=(Buyin w. rake)*(1+ROI) / Prizepool
ROI=(Prizepool*Winrate - Buyin w. Rake) / Buyin w. Rake
Source: TwoPlusTwo
3) Good winrate for HU and 6max SNG?
Well, that depends on many factors (stakes, multitabling, etc.). Lots of regs will argue that for HU a good ROI is around 3-8%, i.e. winrate of 56-60% (win 6 out of 10 games you play for 5+0.5 HEADSUP SNG)
Here is a more general guide including all kind of tournaments (including 6max that you are asking). Stakes you are playing are taken into account, so it states that for low stakes you can achieve a ROI of around 15%, but as you go up stakes you drop down to a ROI of 5%
If you want a good comparison, I would suggest using SHARKSCOPE, search for top-players in your area (like top 10 players in $3.5 HeadsUp sit and go tournaments), check their stats and compare.
EDIT: Forgot to mention that these numbers are for regular SNGs. If you are playing Fast/Turbo/Hyper-Turbos, your ROI might be affected. In fast tournaments you play more games in the same time, so basically you increase the volume at the expense of winrate (to put it simply you win less often, but play more games), so this could work in a either a costly or very profitable way,depending of course on your gameplay.