I learned this from a camera Assistant back in the 80s. Film days using Panavision camera. A 50 mm lens, 5 feet away, will produce a choker close up. If it’s 10 feet away double that. 100 mm, 10 feet away, also a choker. And so on.
And don’t forget to ask the operator “what are you wearing”? If he’s young, he’ll have no idea what you’re talking about. If he’s old-school, you’ll get a chuckle out of him.