[Leica] OT: ground glass query

[editor at frontenachouse.com (editor@frontenachouse.com)]

This is a large-format query more than a Leica one, but since this group 
boasts so many august polymaths, I figured it would be the logical source 
for advice. 

Here's the question: In calculating the distance from lens to the ground 
glass, should the measurement be made from the front of the ground glass? or 
half way between the front and the back? or some calculated plane between 
front and back?  

The ground glass itself is thick enough that basing measurements on the 
incorrect plane would seriously screw up the focus.  

Thanks to one and all who can help me with this.  Rose