TomD
"the "center of effort" could be defined as the height at which the user applies a horizontal force to the plane in order to move it forward along the workpiece."

That is reassuring. I didn't get that at all, but that is what I think it should refer to. Except it is more than height, it is position along the plane, and any vector in any direction off dead straight ahead. Though in many cases the discussion might be simplified to include only height, and a simple forward direction.