I can't point to a formula, but I know that load mass ( note that; mass, not weight ) is very carefully calculated, and a vehicles capacity to tow is an engineering calculation and not a guesstimate.
In this country it is difficult to understand how the authorities interpret the manufacturer's stated limits, but it's important to note that these limits are arrived at through engineering, and the licensing authorities will not accept a manufacturer's stated limit simply on their say so.
A normal car, LR's included, have two stated figures: Maximum total mass of trailer, and max total down on tow ball. This is then modified by the braking component, as Dave said.
The maximum mass will depend on many things. The mass of the towing vehicle, the type of construction of the towing vehicle, the manufacture and placement of the towbar, the type of tongue or hitch, the type of towball, the list goes on. Pintle hooks can tow more mass than a 50mm machined ball ( the old two piece towballs are now illegal ). The square type receivers ( known as Haymann Reese type here, but I doubt they invented it ) can tow more than the old two bolt type tongue we all swore at when we knocked our shins on them.
LR were one of the first manufacturers to include a serious towing package, and we should thank them for it.
Anything with a ladder frame chassis will tow more than a monocoque.
Now, why does a Ford Ranger have a higher capacity than a HiLux? Because Ford has the papers to prove it. Is a Ranger stronger/better braked/ more powerful?? Who knows. But I doubt it. Is a D2 a better tow vehicle than either of them? Almost certainly yes. For now.
