I don't think we can omit water. It makes a lot of difference if you are hiking with frequent water sources (a quart--2 lbs.--at a time is plenty) or where you have to carry at least a gallon (8 lbs.). If I were hiking in the latter situation, I'd go for only 2-3 days. In other words, total pack weight should rule in these situations.

I also think the % of body weight isn't all that great an idea. Those of us who are overweight shouldn't be carrying as big a % of body weight, because we are already overloaded. Those of us who are getting up in years with creaky, arthritic joints and other ailments need to reduce the pack weight as % of body weight. Young children, whose bones, ligaments and joints are immature, shouldn't be carrying as big a percentage.

The length of the hike and of the hiking day is also important. IMHO. If you're going only a couple miles in to set up a base camp, then you can get away with loading up. If you're planning a hike of several hundred miles or more with 20-25 mile days, then a heavier pack (as % of body weight) is far more apt to cause stress injuries even in a young, well-conditioned person.

As you can see, regarding a "rule" or even an "average" of pack weight vs. body weight, I wouldn't want to go there. YMMV, HYOH and all that!

Or maybe my real reason is that I don't really want to reveal my weight after two Christmas parties in one weekend?