I believe, and this is merely my interpretation, the objects front plane minus extrude value is used.
The greater the perspective value the lower the z value for the back plane and the more reduction in scale. So ...
perspective + 1 = ((front plane - extrusion value) + (z - 1)) + ((width - 1) + (height - 1))
Thus moving the back plane further away from the viewer while reducing its size. It is not true 3D, merely a representation which looks accurate.
Since each object is wholly independent, all numbers are based of of the actual object as opposed to any scene or camera setting.
Illustrator's 3D effect has always been "best guess" and has never really been anywhere close to real 3D. Real 3d bases extrusion and perspective on the camera or view angle. Illustrator bases extrusion and perspective on the object and the center of its front plane.