Jim's post reminds me of a little trick I learned from ancient markings on a couple of my old shutters: put little paint/pencil marks on the aperture scale at 1 inch and 1 centimeter. It makes the bellows extension math incredibly simple. 30 cm extension at 1 cm aperture? f/30. 45cm extension at the same aperture? f/45.

For a smaller apertures, a mark for .5 cm is almost as simple.