Murray, when I get back to work on Monday -- home PC is kaput, I'm in the public library -- I'll send you a copy of my handy dandy macro flash spreadsheet. And then you can tear it apart and see the magic formulas etc.

By the way, in it I allow for pupillary magnifications other than 1.0, which is what all of the rules of thumb people throw around assume. If you think that PM can safely be neglected, take a look at the little data sheets Nikon provided with the PB-4.

But on the whole the easiest way for idiots like me to do flash closeup is to build a simple non-adjustable flash bracket and take some test shots with it. I've always done 1:1, 1:2, 1:4, sometimes 1:6 also. Each magnification at every aperture from largest to smallest, usually in 1 stop steps. The last calibration runs I did were in 1/2 stop steps, I don't think that bought me anything useful. For other magnifications, linear interpolation works just fine. Been there since 1971, see no need to change.