Greetings Jack, welcome to the Forum !
Wow, you're really asked a tough one here.
There are a lot of variations which can flex that distance quite a bit. It depends on the stiffness of your Suspension, if you have Auto Leveling, and the Tongue Weight of a Trailer can vary. But the issue is not the height of your hitch, when it's not loaded, but when it is.
If you're Towing "most" rentals, they try to set their tongue height at about 18", so getting a Offset which puts your Ball at that (loaded) height will probably be close.
But, the most important thing is that the Trailer be level, or tongue be slightly lower than that if necessary. If you have your own TT or other Trailer, you can drop by a Hitch Shop, and they'll be able to guide you to which offset is best for "your" personal rig. Also, they make Hitches which have adjustable Ball heights if you intend to tow a variety of trailers.
If you'll tell me just what you're trying to do/tow, I may be able to be more specific.