I don't know of any formula.
However, I vaguely recall that the Royal Mail advise their employees not to lift weights of more than 12.5kg
from royal mail postman/woman job details... 8 heavy mailbags a day (weighing up to 16kg)"
I'm fairly certain that 12.5 kg is the maximum lift recommended by the offshore industry, which I believed until now to be HSE guidelines. However, looking at the HSE website informs me that there is no HSE advice on maximum lift weight/manual handling.
Liability is the buzz-word, not safety, and it's all done with bloody mirrors!
I've carried some horrendous weights when going out on climbing trips; a full rope, harness and ironmongery all make a mockery of carrying lightweight camping gear; and have come across Squaddies carrying similar loads... at times jogging past me. I have mates who are ex-Paras and Marine Commandos some of whom, in their early 50's, can hardly walk without a stick. The difference here being that I carried such loads voluntarily, occasionally and with a dump and return option, while they carried them regularly and often for days on end.
The load you're carrying relates to your purpose. I'll carry some 15kg if out stravaiging the hills for a week in a region with little hope of a fire, although in the Scottish hills fuel is very often more readily available than is apparent to most people. Bog wood occurs in the most unlikely of places and both peat and deer dropping burn admirably when dry. Cooking predominantly on a fire is a means of lightening your load considerably.
Others have stated the importance of the quality of rucksac you're carrying and the loading of it. This is important for comfort, balance and weight distribution. Modern rucksacs have adjustments enabling you to raise or lower it on your back, transfer loading points and easily shift the point of balance when going up or downhill. If you're tempted to strap stuff outside your rucksac this throws the load backwards and results only in giving yourself a harder time than you need to have.
Lastly, my rucksac is full of "essentials" I can't remember the last time I used. Short of discarding vital equipment, if you don't need it don't carry it!
Cheers.