Well, I hate to be the bearer of bad news but unless they've changed how backspace was measured in the last 10 years or so, you've been buying them wrong for 40 something years.
Backspace is measured from the outside lip to the wheel mounting surface. I've bought rims from Summit, I've bought rims from Enkei, I've bought rims from Aspec on eBay. The advertised backspace is measured from the outside lip, just like how you measure it in your garage. I'm quite certain of this, because none of the rims I just mentioned would have actually fit on my car if they'd been off by a 1/2" on the backspace, I don't have a 1/2" of clearance to give.
Proof you say? Here's the rims I bought from Summit, Legend 5's. These are 17x9's with 5" of backspace (0 offset) from my Challenger.
The measurement- You can see the straight edge resting on the outside lip at the top left, and the ruler showing 5" of backspace (well, a smidge over, but the camera angle isn't straight out from the ruler, had to be slightly higher to show the outer edge, the result is a tiny fraction more than 5" in showing in the picture)
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The rim, really is a Legend 5, and you can see the tire's a 275/40/17 so it had better be at least 17x9
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And the specs cast into the rim, yup, 17x9.
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And the ad right from Summit, link here
https://www.summitracing.com/parts/vsw-141h7965gm0/overview/
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