http://www.photostuff.co.uk/pinholec.htm
Having looked at the above website, it does seem to answer your question. It gives the image diameter as output. For the 125mm extension with your F252 which is a pinhole of .5mm, then the output image diameter is 162mm. Well on a 4X5 neg the corner to corner distance is around 162mm. But that won't cover 8x10.
So you need to play with the numbers in the calculator to give an output image diameter covering 10x8 which is around 325mm. And infact, doubling the extension to 250mm does it exactly except that it recommends a 0.7mm pinhole and not 0.5mm.
So I think I may have been correct about the vignetting due to inverse square law but I don't understand why cos its a pinhole and not a lens. Perhaps thickness of pinhole material is a major factor. Probably since I heard the best are made from gold leaf which is incredibly thin but totally lightproof, (except for the hole).
(a pinhole material of 0.5mm thickness with a 0.5mm hole will provide a cone of usebale light which is 90deg before any vignetting due to light hitting the sides of the hole. So thinner the material, the bigger its useable cone of light).
New years resolution: Must stop rambling...