I can't quite see there should be any bellows factor to take into account, unless you try to do macro with it. The film plane may be displaced, but you move the lens end stop back to compensate. So the bellows have the same extension as previously.
EDIT: Bellows factor is when you extend the lens so far out that the real life focal length gets changed. An issue in macro photography, but first coined by large format photographers when using long focal lengths and loooong bellows. But not unlike when using teleconverters. For instance, a 2x teleconverter will require you set f5.6 when the exposure meter says f2.8 (unless of course you have a TTL meter and a teleconverter with electronics, in which case the camera takes care of the calculations)