i'm basing my -40 max on this ...
http://986forum.com/forums/general-discussions/65583-wheel-fitment-question.html
with max wheel width and max tire width (first post is a pic of a 275 tire on a 10" wheel at -34 offset). bit more room on the front (i am at -38 currently on an 8.5 w 235's) but really camber and ride height dependent.
edit to add that it becomes part of the math - if your wheel is a 9" (ie, an inch narrower than mine) then you can add 1/2" = 12 or 15 mm to my max offset. if you are on a 265, then you are 10 mm narrower than me, so you can add 5 mm to the offset.