Originally Posted by Old-N-Feeble
20x is a 'soft' physical limit of the system. Your effective aperture wide open (f5.6) with that lens will be f117 at 20x. That is the same effective aperture as making a 16x20" print from 35mm negative using a 50mm lens set to f8. The effective aperture will determine your diffraction.
Beyond 20x you can eventually get into the range where the diffraction is so bad even with the lens wide open that use of a grain focuser is futile, the grain is always mush. In those cases, standing back by the enlarger and just focusing until it looks sharp on the wall can be the best option. The huge prints can still come out very good as long as the viewer has some distance on them is not looking at them with a 10x loupe.