As my bicycle guru friend pointed out today, it is best not to assume that any reference point is correct in this repair. The Park tool designed for this job puts two planes in the center of the dropouts. If these two surfaces are coplanar, the forks should be correct.
I would like to replicate this tool. Does anyone have any "homemade" versions of it?
Hillrider, You are correct. The tool costs more than my needs dictate. Also, sorry for the misused term for centering the wheel. I refer to "dishing" as any movement of the rim relative to the hub.