Backspacing still doesn't make sense to me.

Yeah, don't try to make it any more complicated than it needs to be. Special tools and measurements for tire sizes are prone to error. You won't really know for sure what it looks like until it's actually mounted.

Go from the known measurements of your existing wheels and reverse engineer it from there. For example, if you have 3in. backspace and 2 inches between the inside wall of the tire and leaf spring you can probably safely use one additional inch of backspace.

If the outside wall of the tire is already too close to the fenderwell you may have swapped in too wide an axle/rear combo and can't go much of anywhere with it unless you're talking about custom-made wheels with offsets that may not behave/handle properly.