. . .can I determine the distance from camera to subject using the 50, then do my calculations?
Using a 50mm lens on a 35mm camera may be misleading because of the difference in aspect ratio, Paul. But if you can mentally crop what's in the viewfinder, it might be useful for a rough guide. The more precise method would be to measure the space, as well as how large of an area you'll be using as a set, how big you want that space to be on the negative, and then plug those numbers into the focal-length/magnification formula to work back to the needed focal length. With a shorter/wider lens (e.g. a 240mm G-Claron on 8x10), you can always move the camera closer. Backing up through a wall, however, often presents more of a challenge.
You can find various formulas to calculate these things in the Lens Tutorial article at:
The shareware Palm program f/calc is also handy.
Another simple approach to determining the desired focal length would be to use a
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, where the opening is the negative size, and the knots are tied at the focal-length distance from the card. Hold the knot to your cheek, close the opposite eye, and look through the opening. What you see is what the film would see using a lens of that focal length. A cloth measuring tape can be substituted for the cord, and used in the same manner. This approach might be the easiest.
I think I'd work backwards in the process to help determine your best format choice. If you determine how your prints are going to be made, and what enlarging equipment is available there, you'll have a better handle on what your practical options are.
You can also use the DOF calculator I mentioned earlier to balance between the DOF you need, and the finite output of the lights you'll be using. For example, assuming that you be using studio-style electronic flash units (perhaps rented for the purpose), set them up as you would for the shot, set them to maximum output, and take a flash reading. If you get a reading of f/16 for ISO 100 film, you can use that as-is, or adjusted for a faster film, to figure the DOF you need. (Remember, with electronic flash, only the f-stop controls the actual exposure.)
For the group shot you mention, for example, having about 4 ½ feet of DOF would be handy. Using the same 12' shooting distance, and an f-stop of f/16 with ISO 100 film, the 240mm lens would give you that DOF, but the framing would be considerably looser than with the 300mm lens. To get enough DOF with the 300mm lens, you'd have to switch to an ISO 400 film. (Although you can build up exposure with multiple "pops" of the flash, doing so isn't practical with people, as they are likely to move between pops.)