This is
not the easiest way, and for the life of me, I can't figure out why all the text books, manuals, etc. keep pushing that method.
As usual, the easy way is the most direct way. Measure the bellows extension, and while you've got the ruler or tape measure in your hand, measure the aperture through the front element. Divide the former by the latter and that's your f/stop. Period. No squaring this, then squaring that, then getting a factor for reducing film speed or figuring the new f/stop.
If you
really want it to be easy, write the measurements of the f/stop diameters on your lensboard and put a scale on your camera bed or rail, (and for style points, do the division on a slide rule!)
(The weird thing is, in all my time in large format, I've never met another photographer who uses this method. Do you folks all just love squaring numbers?)
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