Ralph, you're a masochist. You think harder than is necessary.
Originally Posted by RalphLambrecht
The easy way is to use a flash bracket that gives reproducible camera-flash-subject geometry and shoot a series of calibration shot. One shot at each full stop from the lens' largest to smallest aperture at 1:1, 1:2, 1:4, 1:6. Use reversal film. Then you'll know which aperture to use given magnification and film speed. Linear interpolation for magnifications between the ones used for calibration works well. If the best stop is too small to suit you, put ND gels on the flash(es) and adjust the aperture accordingly. This approach requires no thinking or calculations in the field. Set the magnification desired, look up the aperture to use in the calibration table, start stalking your subject. Flashmeter, calculations, and even knowing the flash(es)' GN(s) are all unnecessary. Suck and try will do the job.
The key to getting this to work is using a rigid bracket. I use two basic designs: Spiratone Macrodapter/Jones of Hollywood macro bracket, essentially a lens hood with a pair of flash shoes; rigid bracket with a pair of shoes that attaches securely to the camera. Both work but if you get the geometry right with the flash-on-lens type you'll get good exposure at the same nominal aperture (as set, not effective) over an interesting range of magnifications. The rig I use with my press and view cameras gives good exposure from 1:4 to 2:1 and the same f/stop set. This by test, first with a flashmeter (less expensive than testing with film if you have one) and then with film (ISO 100 E6).
With my press and view cameras and a single hand-held flash, I use a tape measure and mental arithmetic to find extension, use GN arithmetic with adjustment for magnification to find the right power setting for a Vivitar 283 with VP-1 held ~ 1 foot from the subject. Works very well when a single flash will do.