And there's the challenge - how to measure off the lensboard on a Leitz V35?

I'm not in front of it at the moment to look, but I'm having trouble imagining how...... maybe if I could get a small round piece/disc of glass, put double-sided tape on one side, take the lens out then get the glass disc up in that space somehow, stick it on up there on the lens mount........ worth a try.