It's dependent on a lot of things such as the exercise you're doing, time between the sets, whether you've done any heavy singles to begin with, etc. For instance, with front squats a few years ago, whatever I could do for 5x5 with 2 minutes rest I could do 50lbs more for a single. For percents, that ended up being about 83%. That is just one example though and it could be very different for you.
"Strongman is stupid."
- Mike Jenkins