Almost right, you have to divide the weight by 50 first though and then square root it.
I know the formula! It's the square rooting bit that gives rise to the inequity of charging; the division makes no difference to the eventual outcome.
It's why a Kingair (at 5.7T) pays around a third as much as MOL's 59T Boeing and an eighth as much as Willie's Boeing. Not just MOL that fiddles the figures.
