Bellows Exposure Factors

Hi Experts,

I have an Asahi Bellows unit and have downloaded a great write-up on "Close-Ups". There are tables at the end of the article showing maginifiaction, Film-to-Subject distances and exposure factors for 6 examples of Pentax lenses. It is quite obvious that for a given focal length, the brighter lenses have a lower exposure factor (less light loss).

When using floods, I simply let the Spotmatic meter do the reading no matter what I have hanging off the front macro, tubes, bellows etc.

However, I'd love to take advantage of Thyristor flash guns and now need to compute f stops, and now must understand how to compensate for length of extension and subsequent light loss.

What I can't comprehend is how to actually apply Exposure Factors - which can be as high as X 17.5 for a 28mm f/3.5 Super Tak at 4.8 magnification (longest extension)..

I'm pretty sure I'm to use a Manual setting on the flash, measure the flash-to-subject distance, read the f stop and then apply an Exposure Factor. If the flash (at 100ASA) is telling me f22, do I multilpy 22 by .175 to get f3.85 (for the above extreme example?)

Ow, my head hurts - again.....

Halp!

As person with learning disabilities, I have trouble with math.
For what it's worth. I solved that same problem with a note pad, some slide film and some carefully controlled test shots, which found thier way into a binder. Set up a repeatable shot. use the aperture, then vary you shutter speeds. Use your processed slides for reference, and calculate other shutter/aperture combos for each lens/bellows combo.
Hope somebody else can help you with specifics, I would if I could, I had a photo encyclopedia, around here somewhere....

Wait a minute that was for constant light.
set your shutter speed as constant and adjust your aperture, for testing with flashjavascript:emoticon('')