Wheel Backspacing

I have a 93 Explorer Sport with the factory 15x7 "deer hoof" rims and 235/75r15 tires.
What is the Backspacing and offset of these rims?

I'm looking to upgrade to 15x8 rims with 31x10.5r15 tires (with a 4" lift obviously)
What would the IDEAL Backspacing and offset be for these new rims?

 

[4x4junkie]

Stock 15x7" wheels are 4.5" backspace.

Anything from 3.75-4.5" backspace on a 15x8" wheel works well (might want to use 3.75 if you think you might ever go wider than 10.50 tires, otherwise they could rub on the swaybar and/or radius arm)