I don't agree with the 20% rule.
My first son only weighed 8lb 3oz but that was enough to make the wife whinge for months.
My first son only weighed 8lb 3oz but that was enough to make the wife whinge for months.

My first son only weighed 8lb 3oz but that was enough to make the wife whinge for months.![]()
"I remember having two 3.5 rockets, four 90 (Energa) grenades ... Eight No 36 grenades, six No 80 (white phosphorous) grenades. Five 20-round magazines of rifle ammunition, plus 100 rounds in bandoliers. One 250-round box of .30 calibre machine-gun ammunition ... My bergen rucksack, loaded and ready to go, weighed 98lb. My belt weighed 22 lb. - 120 lb total [without] my rifle. Everyone had similar loads to carry."
Lofty Large, One Man's SAS, pp.66-67
I doubt I ever carry more than 20kg more usually about the 15kg with water, but I recalled watching a documentary about the SAS in Oman and the TV crew took a former member back with his son to the location of a particular mission and he mentioned that he'd had to climb near-sheer cliffs carrying over 120lbs of gear, so I hunted for the quote and found this ...
I think that might be excessive by most people's standards, except perhaps TeeDee's self-enforced 4 miles with the same weight.
From another website...
"The most famous yomp of recent times was during the 1982 Falklands War. After disembarking from ships at San Carlos on East Falkland, on 21 May 1982, Royal Marines and members of the Parachute Regiment yomped (and tabbed) with their equipment across the islands, covering 56 miles (90 km) in three days carrying 80 pounds (36 kg) loads".
heath and safety has become ridiculous, as long as you've got the correct lifting teqnuqie, you get used to the weight and can lift heavier stuff without hurting yourself
One sixth of your body weight or one fifth if you are fit - so I was told. Terribly difficult for me to get everything down to that weight ratio though. I'm pretty close to achieving it for the pack weight but when also adding up weight of clothes, boots, coat and anything in the pockets, I'm way over. None of my calculations include food and water so still working on which 'essentials' to take and what should be left behind. Very tough choices to make ..
"...is there a formula about for height to weight carried?...Not going hiking, as normal, but will have to walk a short-ish distance to site. Looking at a lot of tents etc, and thinking need an idea of what is to much?
And the more hiking you do, the lighter you get, so you have to keep getting lighter and lighter kit![]()
That's an interesting point there, I was reading a book by Ranulph Feinnes the other day, and you have to consider the weight of a man hauled sled, vs the fact that the haulers grow lighter and weaker by the day. These guys are superhuman compared to me. I am currently losing weight and a sixth of my weight would be 27 and a half pounds. About the weight of the average folding bicycle.
I think it does very much depend on how the weight is distributed, consider the average weight of a suit of armour in the middle ages, or what the Roman soldier carried, way more than a sixth I am sure and folk were a lot smaller in those days.