How do you focus your enlarger? -- Just like I focus a view camera!
Method for critical focus when negative is not held flat or lens is not perfectly flat field. Focus the enlarger by moving the head on the column and focus like a view camera. Focus on the corner of the image and note where the column is. Then focus on the center of the negative without touching the focus knob, by moving the enlarger head. Then set the head to the point exactly half-way between the extremes. This will optimize your depth of field at any aperture.
If you want, you can actually calculate the aperture to get it all in focus based on the focus spread (based on view camera focus equation of Hansma).
N = 20/(1+M) * square root of 'dv'
N = Aperture number
20 = user dependent constant (circle of confusion 0.15mm for me)
M = magnification
'dv' = millimeters of focal depth on the enlarger column.
Why convert a simple practical process into a mathematical formula?