Keep in mind that yes...while there is indeed an amount of focus "slop" at the paper (easel) plane, there is practically none at the negative plane - and this is where the DOF in minuscule and yet if some inaccuracy does exist within this upper plane, stopping down can help a bit, sometimes well enough that doing this is well worth the risk of diffraction, although the other expense of possibly very long exposure times (and a bit of attendant reciprocity failure).
My remedy? Use a really clean glass-plate carrier and spend a bit of time carefully aligning the enlarger. Well worth the effort!
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