In case anyone is using a monorail in the field like me (it's not that bad really), there is an extemely easy and totally free method of determining bellows ext factors using the mm scale on the rail. The only calculations are very simple and takes a few minutes. I included my chart as an example:
The inch column is the distance from the lens plane to the film plane in 1/2 inch increments for 6 inches of extension, I may change it to 1 inch increments; the mm scale is the distance from inside to inside of the front and rear standards to equal the lens and film plane distances shown--I can quickly set my focus to infinity by setting the standards apart by 126mm inside to inside anywhere along the rail. The last column is the calculated factor. I always multiply the factor by the shutter speed.
Ex: My recent image in the gallery of the leaf and bark required a bellows ext factor (BEF). I simply read the distance on the monorail from inside to inside of the standards and it was 166mm to equal about 9 3/4" from lens to film, read the chart and used a factor of 1.4x