This is a pretty common question, but I can't seem to find a good thread on it on APUG, so let's make this a sticky and hope everyone chips in to give all the standard answers.

There are several methods for calculating bellows exposure factor.

I determine bellows factor in relation to magnification of the subject. You can do this by putting a ruler in the scene and measuring the size of the ruler on the groundglass to determine the magnification ratio, which I do for still life setups, but for most situations I can usually estimate the magnification of the subject in relation to the size of the format. For instance, on 8x10", a tight headshot--head and shoulders--is about 24" across, so 8/24 is a magnification factor of 1:3. Then I have a table that I keep taped to my light meter, my notebook, and the back of each camera, to convert (and to remind myself to convert) magnification factor to bellows factor. I've posted that table in DOC format here--

There are several copies of the table on the same page, so you can cut them out and attach them to everything.

I like this method because it works well with any format, even if you can't put a ruler up to the groundglass, and I can have a single method for every camera I use.