I have a program I downloaded for my palm pilot that gives extensions, DOF, tilts - a lot of stuff- It is cool but overkill. In 4x5, unless you are doing macro photography, bellows extention is usually one or two stops. With a 210mm lens, I can't ever remember going over 2 stops for still life at a foot or two. You have to be pretty close to even get to one stop of compensation. There is a pretty cool looking tool here:
http://www.salzgeber.at/disc/disc.pdf.
Anyway- bellows extension exposure correction is one of the easier technical challenges I have been faced with. - Frank