You're alluding to the US customary units, but as far as I can tell, at no point was the US foot or any antecedent unit ever based on the length of someone's fo

You're alluding to the US customary units, but as far as I can tell, at no point was the US foot or any antecedent unit ever based on the length of someone's foot, let alone a ruler's foot. The primary length standard at the time of US independence appears to have been the length of a physical yardstick, which first appears to have come into existence sometime in the Middle Ages. What was used to determine the size of the various yardsticks is extremely unclear, but at no point does it seem to have been based on anybody's foot (although an arm is one of the suggestions).