For me and most people I know, everyone has a quite specific point beoyond which the pack becomes exponentially uncomfortable. A friend of same age, weight and hieght as myself has a "comfort point" of 20 pounds. Mine is 28-30 pounds. We both are skinny and weigh about 115 pounds. She has a gimpy neck; I have gimpy knees. I think it is her neck problems that cause her discomfort at a lighter weight. Of course, lighter is more comfortable, but the discomfort is not linear. On long trips I often start out with 40 pounds and although it does not slow down my rate, it is painful until I get down to my magic number. Of course, if I am going to carry 40 pounds, I use a pack that has suspension designed for the heavier weight. With the proper pack, the discomfort does not hit as soon, but nevertheless, it does hit me before the day is done. Below my limit I can carry the pack all day and it does not feel much different as the day goes on.

Thus, I think everyone is different but the 1/4 body weight (if you use "lean" body weight -what you should weight, not what you DO weigh) is a pretty good generalization.

When I talk "pack weight" I obviously mean everything on your back, water included.