If the plane of focus is a half-inch at the easel...

[tlitody]

Depends on the depth of field of your enlarger lens, its particular focal length, and how far it is stopped down. Do you defocus your camera shots by half an inch, or
even a millimeter? Why do it on the enlarger?

Its not depth of field, it's depth of focus at the plane of the projected image. An enlarger is a macro camera. It's taking a picture of the negative. At the negative you have miniscule depth of field. But at the paper you have a much bigger depth of focus, especially when you are enlarging past 1:1. Then the lens extension causes an effective change in the lens aperture. For the parameters given by Bill in last post that would be to around F22 from a set F11.
So using the formula he would get around 8.8mm of depth of focus. (3x enlargement based on 10lpmm). Depth of field at the negative would be about 0.25mm which is why enalrger alignment is much more significant.