Backspacing and offset are directly related. Offset is the distance that the mounting surface is off the center of the rim. Backspacing is the distance from the mounting surface to the back lip of the rim. Here's a chart that explains it fairly well.
A 17x9 0 offset wheel will clear 35x12.5's no problem. If you want your tires to stick out further you could go with a -12 offset. I'm running 20x9 -12 offset wheels with 35x12.5 Toyo MT's and I stick out 2" past my fender on the front. There's a link to my wheel/tire thread in my signature below with lots of pictures if you want to take a look.