It took me a while yesterday to realize why I facet, because I have long since forgotten.
I've never used a specific radius, it's a nuisance to find something the right radius or make a template or whatever, and I'm always looking to make things, not make templates.
I recall at one point applying a template to trace to the end of an iron, and I found it cumbersome, so instead of doing that, I guess about where I want the depth of the camber to go to, mark it on both sides, and then cut the facets.
If Chris Schwarz says he uses 9" radius for camber, I don't really care what he says, because he's an editor and printer. If warren knew the number he had, I'd certainly pay more attention - but he may work different stuff than I do, or work slightly differently with his jack plane and prefer something different.
I'm not getting to the point well here, it's two fold.
1) the faceting is a quick method, you don't actually have to lay out the whole curve, just the facets and then round them, cutting to depth at the corners and none in the middle
2) evaluate in use. If the camber is too much, hone the center a bit hard the next few times you use the iron and grind biased toward the middle. If it's not enough, do the opposite
It's pretty hard to *not* figure out what works best given the effort in hand dimensioning, unless you only ever do a little bit. If you do a significant amount, you'll figure out what works best pretty quickly, even if you only have a small amount of insight.
(the narrower the iron, the more difficult it becomes to trace a template, or set one up. Making the hollow and round irons comes to mind. Fortunately, if you don't do a perfect job with them, nobody will know unless they come to one of your mouldings with a template).