Something else that might work is to use your rear bumper height as scale.
Say you want a 600# tongue weight. Measure your unhitched bumper height with three 200# people standing on the bumper. Then lean a yardstick against the bumper, hitch up, and drive your tractor on until the bumper sags to that 600# height.
I wouldn't extrapolate the 600# height to a much heavier tongue weight, in case your rear spring rate is non-linear. And next time, your bed should have the same loading as when you took the 600# measurement, if your accomplices aren't handy.