Dave, I think I see where we're diverging.
What I'm talking about is measuring rider sag only (bike+rider). For that single measurement, spring rate doesn't matter.
I think you're also considering free sag (bike only) and comparing the two sag values. In that case I agree with you that a progressive spring is more confusing to calculate than a straight rate spring to get from one sag number to the other. But I guess I'd also claim that even if you know your weight, without knowing the front/rear weight distribution of the rider's weight, knowing the exact spring rate isn't all that valuable either. Also, I've seen many sag setting articles that don't even quote a free sag spec, just the rider sag. The main thing about free sag is you want a little travel to remain without your weight on the bike so the suspension won't top out too easily, but I don't think an exact number is terribly important.
So in my comments I'm thinking along the lines of setting rider sag by adjusting the spacers as I described, then after that is set take the free sag measurement, and if its in a reasonable range, leave it alone and don't mess with anything else. I suspect you're trying to dial in both numbers a bit more accurately than I am, which may require a bit more fiddling and calculations, in which case you're right about the spring rate having an effect.