Interesting question, but from my perspective it barks up the wrong tree, unless you are wondering what weight would actually damage you if you tried to carry it.
When I was 17 I weighed just under 130 lbs and did a multi-week hike where I carried a bit over 60 lbs on day one. It didn't kill me or even damage me, but it did slow me down considerably. Would I recommend this to anyone? Never!
My point is that the ideal weight is always going to be determined by the requirements of the trip, not by some magical ratio to your body weight, no matter how you measure it. All else being equal, the least weight you can get away with is best.
I now weigh 160 and typically carry about 32 lbs if I am going out for a week. I find this very comfortable to carry, but no doubt if you forced me to play the mule and carry 50 lbs, I could drag my sorry behind up the trail a few miles and shout hallelujah when I disburdened for the day. It wouldn't kill me, but I wouldn't enjoy it.