Sorry, I misread the f/calc result. That should be a quarter of a foot or about three inches, rather than a quarter inch. I've corrected the post above.
Also DOFmaster is rounding the 8x10" CoC value (.2) more than 35mm CoC (.03) just following the normal rounding conventions, but that aside, the most common 300mm lenses for 8x10" aren't wide open at f:8, so at f:8, there is usually room to spare. The exceptions would be Artar and process type lenses or compact lenses like the Fujinon C, which are usually around f:9. Most 50mm lenses for 35mm cameras are f:1.4 or f:1.8, so even if an f:1.2 isn't quite as exotic as an f:1.0, they're still pretty unusual.