STOP!
Your over analyzing things to death.
Load the truck like your going camping and go to the scales. Subtract your scaled weight from the trucks GVWR. Thats your available payload.
TTs will drop approx 12% of their actual weight on the hitch.
Fivers will drop approx 20% of their actual weight directly over the rear axle. Of course you need to reduce hitch weight from your available payload.
Overloading or exceeding your payload is not illegal, but pretty stupid.
The advertised dry weights are also meaningless. You will never tow a dry trailer. Instead look up or find the trailers GVWR, and use that. You may never reach the trailers GVWR, but any less will give you a margin for error.
