There is no rule of thumb. In fact it is very rare that a TC does not exceed the factory payload rating of the truck carrying it.
If you come anywhere near your truck's payload capacity, you will usually feel the need for some sort of suspension help regardless. This is to reduce the rear end sag that you normally wouldn't worry about if you were, for example, hauling a pallet of cinder blocks to the job site.
What many people do is go by the axle weight ratings of the truck. That's usually limited by tires on the rear axle, so if someone is wanting more, they simply upgrade the tires and add more suspension modifications.
So in a nutshell the ratings don't mean a thing. Just do what feels good, and if it don't feel good, throw parts at it.
I'm one of those rare occurrences where I'm not exceeding any rating on my truck. By a lot. Not by choice, necessarily. I was looking for a 2500HD truck, and this one fell in my lap. I was looking for a larger hard side camper, and this one fell in my lap. Been running this rig for 5 years and it does the job. I'm not fussy, and when I'm not camping the truck will tow or haul just about anything.
Putting 10-ply tires on half ton trucks since aught-four.