trying to grasp guide numbers... question?

[Garbz]

So the variables are, ISO, Aperture, Distance flash-to-subject, Power, Flash Zoom. ...

I think this is an area where it's easier to take a test photo and then readjust rather than do photography by the calculator

 

[Derrel]

I think the exposures on these faces look pretty good; perhaps a slight bit under-exposed, but close enough to work with. The side/separation flash looks a bit too hot on the ears, and also it looks "colder" in terms of color temperature. Warming up the separation/sidelight with a slight warming gel would be one possibility. There's an old sort of unwritten rule that the fill and accent lights ought to be warmer in color temperature than the main light's color. If a speedlight flash is used with ,many shoot through or reflective umbrellas, there's a good chance that the umbrella's fabric will warm the light up, and any light coming from a bare flash will be much cooler.

You know, it's possible to get a 3:1 lighting ratio by placing a main flash at say 5.6 feet from the subject, off to one side and angled, and then to position an identically-powered flash right next to the camera lens, but fired from 8 feet. It might be surprising to you to find that 2.0 feet, 2.8 feet, 4 feet, 5.6 feet, 8 feet, 11 feet, and 16 feet are intervals of distances where flash varies in power by one stop between each neighboring distance...

 

[mikehaugen]

You know, it's possible to get a 3:1 lighting ratio by placing a main flash at say 5.6 feet from the subject, off to one side and angled, and then to position an identically-powered flash right next to the camera lens, but fired from 8 feet. It might be surprising to you to find that 2.0 feet, 2.8 feet, 4 feet, 5.6 feet, 8 feet, 11 feet, and 16 feet are intervals of distances where flash varies in power by one stop between each neighboring distance...

That's interesting, I may have to experiment with that. That could be some awfully useful knowledge.