John. Having read your above, I can also add, that I dont have "an issue" with the diagram or the concept that you are showing me, it is just not what I am talking about. If you placed two scopes of the same dimensions, other than the tube diameter, on a flat surface, with no mounts, then they would sit identically on that surface, and the centre of each tube, would be identical as well. Quite clearly though, you would need different sized mounts, to set them on the rifle in the same position. The height of the centre of each tube would then still be the same, as to would be your head position