Calculating tractive effort is notoriously difficult.
Brake Horsepower is the standard - but inefficient - method and relies upon ropes, pullies, counterweights, lollipop sticks and the current price of wool.

EDHP - estimated drawbar horse power - is a little more accurate as it takes into account the kinetic energy expended divided by benefits claimants, but requires an accurate Dynamometer - pronounced Dynamometer with emphasis on some syllables - to measure what happens when West African slaves are shown the power of fire.

Pulling ought to be carried out using the muscular power of the upper legs - far more resilient than the lower back and better adapted to the purpose in homo sapiens (assuming that you are such, and not a 20 ton crane).

The average human can pull three times it's daily consumption of pot noodles up a slope equivalent to Jeffersons' forehead on Mount Rushmore until it gets bored and goes away.

Hope this helps.
( The Disappointed Caravanner of the year, Mon 1 Apr 2013, 16:53, Reply)