the depth of field calculator on my combat speed graphic with a 127mm optar (ektar) uses a 1/300th of an inch circle of confusion assumption. With that it says, for example, that you are in focus from 21feet 6 inches feet to 29feet 7inches when the lens is set at f 4.5. It doesn't say how large a print that assumes, however, but I would guess an 8 by 10 inch. The larger the print, the less depth of field is safe to assume, as others have said.
I'm something of a pragmatist about these things -- or maybe subjectivist is a better word. Play around until you find a method that gives you nice images and don't worry about the technical aspects of things.
The problem I see with the fixed number model examples above, 1/300 or 1/1000, is that only the lens and film are considered.
Its a guess, but I think a reasonable one, that those numbers were generated based on "normal" print size, viewing distance, and expectations.