I'd go with the tread width. The tread is what will most likely toss debris, water, dirt, etc up into the air and at the vehicle behind you. Using the tread width would give you a little bit more room to play with as far a the size of tire you can put on (example - for the factory BFG 255x17 MT tires on my Rubicon, the tread or knobs are recessed about 1 inch from the edge of the tire at its greatest width).

Also, look at some other rims with a larger numerical backspace number. The AEV rims, as an example, have a backspace value of 5.2 inches with an offset of +10 mm. You would have to run this thru you spreadsheet (or whatever you are using to do the calculations). Viewing the AEV galleries on their website show many vehicles with 35 inch tires with the tread essentially contained within the stock fenders.

You can fit 285x75x17 tires on the stock rims. You will most likely need wheel spacers if you push the suspension off road.

Have fun,,,

 

Hhmmmm "as far as practical" well fenders really get in the way as far as your view goes when looking out the side in the rocks. Big wide fender makes that very impractical for rock crawling. I'd give em 2" out from the body panel