Since this is a sticky thread and since I do not see my particular solution (which I thought was standard procedure...), here goes.
1. Figure out bellows extension factors for all your lenses for a set of exposure factors (I use 1.5, 2, 3, 4, 6, 8, 12, 16, and 20, dropping the higher ones as the bellows length is exceeded).
2. Make a chart and carry it with you in the field, along with a small tape measure.
3. When you need to apply a bellows extension factor, measure your bellows extension, look up the factor in the chart for the lens you are using and apply it as you would a filter factor.
Voilá! Done and no fussy calculations needed in the field.
FWIW, I'm attaching the bellows extension factor tables for my lenses for up max bellows extension 16 in/400mm in both inches and mm. Maybe they will be of use to someone.