Quote Originally Posted by jasonclements View Post

your help with my project has been greatly valued, i can't thank you enough. when you are measuring this distance, what reference points on the lenses are you using to calculate the measurements? The front of the lens? Or the optical center? If it is the optical center, than how does one determine the optical center?
Adjusting this sort of two element lens by precision measurement of spacing is impractical. The nominal dioptre powers are close but not exact. In practice the spacing is adjusted until the image is as sharp as possible for the existing subject and image distances.

The "optical centre" of a lens element is a difficult concept. Real lenses of real thickness have two principal planes each defined by an array of principal points. The two principal points/planes exist because light can be sent through a lens from either direction. One could think that the intersection of the optical axis and a principal plane is an "optical centre" but then there are two "optical centres". This esoteric stuff is irrelevant for a lens focussed by adjustment.