I get that with manual on camera flash you set iso and aperture and distance to the subject. Can gauge distance by focusing and checking distance scale. Easy. But what if it’s a manual flash, handheld, pointed at a ceiling or side wall for bounce, how do you set the distance then?
You can point map out the light, multiply by the albedo, divide by the radiative emissivity of the surface, correcting for temperature variations, and then disentangle the surface roughness, ending with ray tracing the turbo encabulator.
But most people find the math and coordinate transformations in the frequency domain to be too difficult to do in their head, and inputting everything into a slide rule is really slow.
It’s probably fastest to use the guess-and-adjust method the other person said.