There are various ways to calculate the value for acceptable CoC, but the standard values vary by format. Bigger format, smaller enlargement factor for the same print size, so a larger CoC is going to be visually acceptable.

Now, if you're like me, you might want things a hair sharper than the standard values, and rather than enter into a complex calculation, I do exactly what Bill Burk recommends above, with the same result: I stop down one or two stops from what the scale or table says to use for the DoF I want, and I don't really worry about diffraction, unless I'm shooting macro at fairly high magnification, where the effective aperture is going to be significantly smaller than the value engraved on the lens. Inadequate DoF when you need it is a bigger visual distraction than diffraction at non-macro subject distances.