How to calculate bellows extension from the bellows factor?



I sometimes like to play "arm chair" LF using my ancient Palm Pilot. I run pCAM (now a neat iPhone app it seems) and Bob Wheelers extensive Vademecum.
I am thinking about using my Wolly Raptar 480mm for some (sharp) portraits on 8*10. So pCAM gives me the distance from the lens to the sitter (a shoulder/head portrait) through it's nice schematic scene preview function, and the DOF function of the Vademecum gives me the DOF limits and the bellows factor (the correction in stops or time to compensate the bellows extension) Now my question is how I can get from a Bellows Factor of 3.2 to the extension or total length of the bellows for said shot? yes I know I can mount my camera with that lens and so on, but I still would like to know how to calculate the bellows draw in a given situation, also when I am considering buying a new lens, for which I do not have enough bellows to begin with. thanks in advance, Best, Cor Read responses in largeformatphotography.info
