When choosing a stop for my long volatility ETN, I merely go to morningstar and get the NAV, then I put the stop in at real close to that value. It has been between a 4 and 5 percent premium, so I limit my loss to that. It seems to work recently.
The premium you pay is going to evaporate for sure in little time though if things go sour.
Disclosure: I am long TVIX.