lol. Jack- you are the mathemetician! You have figured a forumla for every backpacking problem. It is probably a good rule of thumb for mid weights as long as the pack actually fits you. I suspect it may break down at the ends of the distribution. I use my external frame Kelty for heavy weights. It seems to carry 70 pounds as comfortably as 50 pounds. I definitely feel more weight on my legs with 70 pounds but pack comfort is about the same.

I think packs are like hiking shoes. The three most important things are fit, fit and fit.