On line calculator for hitch extensions

I am not restoring rear axle weight, only the front back to unloaded values.

That would make higher end calculations, but if you have 300lb tongue 8 ft behind the axle, to restore initial axle weight you are putting 2400 ft-lb torque on your frame.
This is pretty serious number.

It looks helpful in showing how much your front axle unloads without a WDH, but with one engaged does not reflect how tight or loose you run your spring bars. My WDH is set to restore all the weight back to the front axle as if I was not hitched.

Very useful IMO.

Good find.