My 16yo son is interested in getting into multi-day backpacking. We have just barely started to do any research, but I have a question I'm just curious about. I know there are all kinds of situational differences, but is there any rule of thumb, a ratio, of body size to the loaded weight of a backpack? For example, my son and I are both in pretty good physical condition. I am 5'4" and 110 lbs; he is 6' and about 180 lbs. In general, how much weight might we expect to be able to comfortably carry? Thanks.