Badger,
I know what you mean. I've always sensed that the first brace is 'the point of no return.' Something that as helped me is to compare readings from finished bows that are similar and then use arithmetic to figure out what the weights should be. If you have a finished bow that is similar to what you want the bow you're building to come out as, you can weigh it with a long string to get an idea of where you should be. Does that make sense? I'm trying to figure this stuff out too.
I think Jim Hamm's chapter of tillering is better than just about everything else out there, but he fails to point out that a bow that is braced too early is going to lose some spring. He seems to advocate stringing as soon as you can--even when the bow will pull full weight at 12" of draw. To me, that's bow abuse. If you want a finished bow of 50# @ 28" and when you brace it, it's drawing 50# @ 12", you are way over-straining your bow. Ideally, a bow shouldn't be stressed past it's intended finished draw weight (as Jim says) or past it's intended finished draw weight for every inch of draw. In the perfect world scenario, if my finished 135# warbow is going to draw 15# @ 2", I wouldn't want to ever push it past that while tillering. Now, does that make sense! According to my reading of the Hamm method (which again, I think was revolutionary) I could string it at a point where it's drawing 135# @ 2" then remove some wood so I could get it to 135# @ 3" and so-forth.
J. D. Duff
PS: I like to see my bows take a little extra set after I've used them for a couple days. To me, this means that my tillering was even more gentle than it needed to be. Am I right on that?