Many thanks, I will look into that, though I am an Android fan (hopefully there is something equivalent).
I am after something like this:
http://www.cookseytalbottgallery.com/bellows_compensation.php
I have found from Ken Rockwell (I know... !):
"'m cheap and good with math, so I calculated and drew my own scales. You can find the formulae in Ansel's Book "The Negative." You read from a different scale for each lens. I draw these on a ruler I lay on my focusing bed, and read the factors from my custom scales. "
http://www.kenrockwell.com/tech/exposure-large-format.htm
I do have "The Negative", (in my edition, pp 65) he mentions: Extension ^ 2 / Focal Length ^ 2 (with whatever other filter factors).. Quick and dirty look last night.. is that it?