I just pulled out my copy of Complete Walker IV, and I'm afraid there's not a lot of help there:

"Once all these ponderables have been duly pondered [adjustments to the rest of the suspension], then it's time to think about bending the stays (if they're aluminum, that is). Some stores have stay-bending experts, to save your trying to do it by yourself. But here's the drill. With the pack on and weighted, feel where the bottom ends of the stays rest on your back. Then take them out, which might take some head scratching and a few grunts. If they slot into the hipbelt, you should take the hipbelt off the pack, slip the stays in, strap it on, and scrooch it down a bit at the sides to simulate the effect of a load. A properly bent stay follows the contour of your back, which is hard to see from in front, so get help or at least use a mirror. Bend the stay over a rounded surface - your thigh or the back of a sofa. The bend the other stay to exactly the same curve and reinsert. Test by walking around with a load. In the pack, that is."

That's it, for whatever it's worth.