Not sure if anyone said this already, but bellows extension factor is pretty easy, once you realize that the inches of bellows draw are in the same relation as f-stops. All you need to know is the length of the lens in inches, and how far out the lens is racked.

For example, say you are using a 210mm lens, which is approx. 8 1/4 inches. Convert the inches to an f-stop, and round off to the nearest stop or 1/2 stop. In this case, your "base" exposure is f8. If your lens is racked out to 11", then you need to increase the exposure by one stop, the difference between f11 and f8. If you're racked out to 16", then it's two stops. For distances in between, approximate. Racked out to 13"? Well, that's f11 + a third of a stop or so. So you'd increase the exposure by about 1 and 1/3 stops.

Close enough for government work!