I square the lens focal length, then divide this into the bellows draw squared. This yields the exposure factor. With black and white, it is easy to arrive at a close approximation of the exact answer by doing the work in your head, which is easier for me than carring one more thing around. If you just use a lens or three, after awhile all you need to do is measure the bellows, and you already know the answer.

With transparencies an exact answer is appropriate, so I work it out on paper or use a calculator.