The edge of tire and and edge of rim are directly related.
But is the thickness between the inner bead seat and inner Flange always the same or can it vary with the wheel design?
Also same width tire overhang over the rim edge will vary with rim width.
Or you could just say 4.5" backspacing or less and be done with it
At what rim width?
For the purposes of clearance backspacing is of more use. All you do with offset is work out the backspacing anyway and you need to involve rim width. You can say that 4.5" or less backspacing is good, there is no offset figure or even a range that you can say is good as it depends on rim width. Much easier to talk in terms of backspacing.
I beg to differ, You don't really need rim width or backspacing when using offset.
With offset I just need to know tire width and one can relatively quickly calculate the approx. inboard position of the tire from the Hub surface and compare it to stock.
e.g. stock 255/75R17 (32"), Moab 17X7.5" , +44.45mm offset
255 / 2 + 44.45 = 171.95mm Approx. Tire inboard position from Hub.
now 315/75R16 (35") on a 16X8, 16X8.5, 16X...., 0mm offset rim
315 / 2 +0 = 157.95mm Approx. Tire inboard position from Hub so about 14mm more clearance inboard vs. stock which should be just OK with the 3" larger diameter tire but safer would be a -10 or -12 offset which would give about 24 or 26mm more clearance respectively (315 / 2 - 10 = 147.5mm).
IMO not so simple and more variables involved when using backspacing but feel free to prove me wrong in that regard