Note that with ULF such as 20x24 inches, the scale of reproduction (inverse magnification) is going to be small. For example, if you are doing a head and shoulders shot with a field-of-view that equals 20x24 inches (not unreasonable), the magnification is x1 and the required bellows draw is 2x the focal length of the lens. With smaller formats, we only use bellows > 110% of focal length with "macro" shots, but "macro" is with reference to the film size and can be hit easily even at a measly 8x10 inches of film.