..well, there isn't very much that one needs to be terribly precise
about except two things:
1. Be absolutely precise about not allowing any light leaks.
(no problem, you say. Well, now that I've given you a false sense of
confidence, I'll tell you about the real difficulty...)
2. The spring back which holds the film holder has to be made *very*
precisely - an error of 1mm. would be bad. You need to be absolutely
certain that when you put a film holder in the back that the film in the
holder is now located in the exact same plane as the inner surface of the
ground glass - otherwise you will have focusing errors.
but if you can master this, you've got it!
dave