The graflex.org site has instructions on how to calibrate a
couple of different rangefinders that might help in a
non-camera specific sort of way as well.

Dave wrote:
I have just bought 2 external rangefinders to use with a newly acquired (for
curiosity and fun) 6x9 format Bessa 1.
One's a Voigtlander and the other a Watameter.

Anyone know how I can adjust/calibrate either of these, or point me in the
appropriate direction for information, even another newsgroup.

Thought I would try with you guys first (I will be using them on a MF
camera) as on this occasion Google hasn't been my friend.
Thanks
Dave