Painting with light
1. Flash is the only light source that will give you enough light of the right color, afaik.
2. You can figure the ambient exposure with a light meter or ttl meter.
3. The flash fill/accent lighting you figure with the guide number for the flash.
4. There is an aperture system called "US", which as I recall stood for "Universal System". I think it is f/16 is the same as US16 but the two systems depart from there. Anyway, it is additive. You can figure the f/stop for each of several flash units, then convert them to US, add them up, take the number you get and convert it back to f, and you'll have your exposure for that setup. Works like a charm but I don't know where to get the US system anymore.
5. Painting with light outdoors is tricky, unless you have a scene that you can get the light on. It is great for interiors, though. The way you do it, is you take an incandescent photo lamp, tape a large black shade all the way around the reflector so you have a kind of long shade on the reflector such that you can point the light at nearly 180 degrees, or at right angles to the camera. It is never quite that much but the closer you can get the better, as you won't be shining your lamp into the lens while the shutter is open.
5a. You figure your exposure in seconds that you can easily count off. Say you have got 10 seconds with the light pointing straight at a central object in your scene. You open the shutter, walk out into the scene with the lamp shade never pointing at the camera, and being very careful not to have any shiny surfaces to reflect back into the lens. Use dulling spray on any of those if possible. Also wiggle your tail constantly. I.e., don't let the lamp power cord stay in one place more than a second out of the 10. You keep moving all the time. Shine the light under the table, constantly moving as you count to ten. That means YOU don't stay in one place, either. Move around inside the scene and paint it all to get the same exposure everywhere, including on the back walls.
It takes practice, but it is easy once you get the hang of it. Probably a lot easier with digital because you can check your shots instantly. We used to have to go back to the lab and develop the film to see if we showed up as ghosts in the scene.
It really isn't by guess and by gosh. You count. Practice makes perfect.