backspacing

I currently have 15x8 rims with 31x10.50's on my 97 XJ I want to go up to a 33x12.5 on a 15x8 rim but I'm not sure what backspacing I need on my new rims is there a estimated amount that most use? Or do I have to measure and if I have to measure how is that done?

I have 4" of backspacing, and they stick out far, here's a pic.

http://www.azvjc.org/pics/5799_0442414334.jpg

I think 4.5" Would work. I saw A set of 12.5" tires on stock wheels one timeAnd you couldnt get your finger in between the tire and leaf pack!! NOT GOOD. I think stock is 5.75" So That is why I say 4.5" but keep in mind I do not know that from experience. 4X4XFAR

I went with 3.25 but could do with less.