So when you moved it so it would cover, you've changed your F to an arbitrary value. Then, you might as just poke a hole and see if it works, because you've invalidated the calculations. If it is supposed to be 76mm, that's the value of F regardless of whether it covers or not. Of course, if you change it to an arbitrary value and can accept the results, it's ok, but you will need to refigure your f/#.

I've often used the formula in the old Photo Lab Index since I started doing this before Eric published his data. That formula is sqrt( .00007 x F) all in inches. I suspect the formula cited above, by Lee, at .000055, would be as good or better. The difference in the various formulae stems from the specific wavelength of light that the individual chooses as a basis for the calculation. The result is always a compromise between the softening effects of diffraction (becomes more pronounced as the diameter decreases) and resolution (which becomes more pronounced as the diameter increases). Any diameter within the range between Lee's quotation from Eric and mine ought to be good. As for the wavelength of the light, I don't bother with it, although I suppose I ought to consider it.

The other factor which nobody seems ever to consider (although grahamp alludes to one aspect of it) is the quality of the physical hole. You can have a hole that agrees with the math and still produces crummy images because it has ragged junky edges and most likely forms a rough tube through metal that is too thick. A nice clean hole with sharp edges will give the best results.