just remember:
smaller number on backspacing means tires and wheels stick out farther.
negative offset pushes tires and wheels out farther
stock wheel backspacing is around 5.5 inches. Wheel width is 7 inches
so for example, you want a 10 inch wide wheel with 3.75 backspacing. That means that the wheel will be pushed out 1.75 inches because of backspacing (5.5-3.75=1.75)
and another 1.5 inches due to the width of the wheel. (10 inch wheel - 7 inch wheel = 3 inch difference/2 = 1.5)
Add it up and your wheel sticks out 3.25 inches farther than stock.
Thats just the wheel....that doesn't include the width of your new tires, which are wider than the wheel.
This is how I understand it. Someone else chime in if you see anything that looks wrong.
IMO, I would stick with a wheel width of 8 or 9, and try to stay in the 4 range on backspacing. Thats what I did.