First question is, so far, unanswerable directly, generally, or universally.
I'm not sure I understand that statement, Ian. I assume you are talking about the OPs opening question.
I think there is an answer. Simply put, you measure the throat and measure your bullets. The universal purpose of the throat is to allow a loaded cartridge to chamber without resistance from the bullet.
I see that replies here are for both revolvers and rifles. But there are some qualifying statements that are missing that would allow an appropriate answer. Maybe that's what you are getting at. We don't know enough to answer the question.
I would think that to answer the question, the following would info would be required.
What velocity are you wanting to shoot
Gas check or plain base
Powder coated or plain cast bullets
Bullet alloy type
if it passes or has a slight drag ands is at or larger than bore Im good to go.
I suspect you mean groove diameter. The throat will tend to be larger than groove diameter. If it was a bore diameter, it probably would not chamber.
For gas check bullets my general rule, which has worked so far is for GC bullets, 0.001 to .002 over groove diameter. For PB bullets, groove diameter. I don't use PC so can't speak to it. I tend to use 20:1 for GC and 30:1 for PB.
On a rifle, to assure the bullet enters the rifling straight, depending on the rifle, you seat the bullet so that it either touches, slightly engraves or has just a few thousandths jump to the rifling. For a revolver, you are stuck with the bullets having to be at least flush with the end of the cylinder face.