When you extend the bellows on the on the Mamiyz RZ67 there are curved lines corresponding to each focal length lens so you can find the focal distance. However, there is also a ruler along the top that measures how far you have extended the bellows in millimeters. Is there a formula using this ruler to precisely determine the focal distance? I'm trying to figure out some formula that incorporates: *focal length in mm *bellows extension in mm *focal distance in mm For example, with a 90mm lens extended 8mm it is focused at 1.2m (1200mm). What is the formula that relates 90mm, 8mm, and 1200mm? Thanks!