I think my wheels are about a 0 offset for 8.5" width, which is about 4.75" backspacing. Add a .75" spacer brings it to 4" of backspacing.
I think people normally run 1.5" (or 1.25") wheel spacers to bring stock to 0 offset. Add .75" to that and you'd need like a 2.25" (or 2.00") to have an equivalent offset to mine.
With that being said, I really wouldn't recommend running 12.50's on a stock width (7.5") wheel. It's just too wide of a tire for that wheel width. I don't think any tire manufacturer would recommend it. Not that you couldn't mount them, but I think if you were planning on doing any sort of wheeling, it probably wouldn't be a good idea. Truth be told, I really wouldn't recommend 12.50's on these trucks period. It really is just too much tire. I had rubbing prior to the little spacers. Some on my UCA probably at full droop, so that was rare, and a little on the frame at the rear of the tire, which happened at full lock and compression. Now after the spacers, I don't have any more UCA rubbing obviously, but I do still get frame rub, but only on the passenger side at full lock in reverse. But what has happened now is that my body mount rub has gotten far worse. Almost violent at times, to the point where its ripping the tread blocks. So I haven't found the ideal offset to fit these tires, nor am I sure one actually exists. I think if I'd have gone with the little 1/4" spacers just to clear the arms, it may have been better clearance wise than the route that I went, but then I'd have to live with a shim style spacer. I can still go that route though as I've got all the parts (spacers and long studs) to do it. I plan on hacking more out of the body mount as well for now.
Now Mud Grapplers aren't your typical 12.50's, they actually run very wide (12.8"). I think other tires will have a narrower cross section like say KM2's for example. So YMMV. You can find actual tire cross section widths on the manufacturer websites.
My next set of rubber will not be 12.50's for many reasons. They don't fit, gas mileage sucks, they track like crap on uneven roads. I'll be looking at anything from 255's to 285's for the next set. 295's and 305's aren't off the table completely, but 12.50's definitely are. It just makes more sense for me on a truck that sees daily driving and gets to see the dirt like twice a year. Something lighter, narrower, and quieter are in my future. Hope that helps.