There's no set formula save for the solid recommendation to stay below the rated towing capacity of your vehicle as a conservative yet competent upper limit.
And depending on the basis for that limit, is also dependent on how much "better" it tows.
IE: your truck, gasser, the limit is more based on the the truck's power to move the load well. At your limit, it will not have the power to pull grades at the same speed as a diesel and not as good of grade braking.
Diesels limits are generally the chassis rating and at the (higher) limit of a diesel (apples to apples trucks), the limit is generally based on the chassis limits.
"Better"? Again subjective. Generally GNs and 5vers are more stable than conventional hitch trailers. And they can generally be turned tighter and backed into tighter spaces than the same length bumper pull trailer.
Putting on alot of miles, I'd rather have a 5ver. Occasional towing, I'd take the $ savings of a TT over a comparable size 5ver.