
It's not that difficult, folks! It's easy to figure bellows factor to within 1/3 stop. We'll take an 8x10" camera with a 12" lens.
Let's assume that calculating within 1/3 stop is as accurate as your exposure is likely to be or need be in any case.
At infinity, the distance between the film plane and the aperture diaphragm is 12".
If you're focusing at a relatively close distance (say 6 feet), you extend the bellows beyond 12". Measure the distance between the film plane and the aperture diaphragm. Let's say it's 16".
The inverse square law tells us that a bellows extension of 24" (twice the focal length of the lens) will require two stops of exposure compensation (intensity of the light is quartered). 18" of bellows extension (1.5 x focal length) will require 1 stop of extension compensation (intensity of light is halved). Our 16" of bellows extension is about 2/3 of the distance between 12" and 18". We need 2/3 of a stop bellows compensation. 14" would give us 1/3 stop compensation. You can make a chart very quickly this way if you need.
14" = +1/3 stop
16" = +2/3 stop
18" = +1 stop
20" = +1 1/3 stops
22" = +1 2/3 stops
24" = +2 stops
28" = +2 1/3 stops
32" = +2 2/3 stops
36" = +3 stops* (* the math whizzes in the group will say that this is not entirely accurate, since the inverse of three times the focal length squared is 1/9th the light transmission and not 1/8th. Open up a smidgeon more.) I figure your camera probably doesn't have more than 30" of extension, so you're probably safe using this chart. If you're doing extreme closeups, use the measuring system in the download provided above. It's more accurate.
The math is not that tough. I never got better than a "B" in math after I was introduced to algebra.
Peter Gomena