I own a shop, and you go by what is stamped on the rotor. You may have to look for it, but it should be there somewhere. Depending on the brake set up, as well as the calipers it has, will dictate how low they can be turned. I wouldn't worry about how low they can go, as much as I would worry about "will they break", and cause you to wreck? When they get too low, they can snap right off the hub, and cause all kinds of problems. Most of the time they will do severe damage to a lot of other components when they do break, and cost a lot of money to repair the damage. It's better to just replace them if they are out of limits on the number stamped on them.
Glad to help out, Good luck!!!
