This is driving me nuts.

I have several cameras with zone focus and I am trying to check focus on them by putting ground side of a piece of ground glass against the film plane. Only one has a stepped plane, apparently one level for film and the other for paper backer. On that camera the GG went against the smaller, 'furthest in' plane, closest to the lens. It seemed apparent that was where the film would lie. I hold the GG flat with a wide rubber band.

Taking the outer cell off to clean it, I learned ther are apparently 3 starting threads, I assume roughly or perhaps necessarily 120 degrees apart. Not only are they frustrating to restart, I can't find any distance yet that makes ANY sense at all. With the infinity 'stop' pin removed to allow removal of the outer cell, I noted that I don't really know how many cell turns in infinity should be at...if it's possible to go one turn too far, or if screwed all the way in is infinity. If I start the thread that aligns with the infinity mark (a good sign), it threads in 2 or (I forget at the moment) 3 full turns. But focussing near or far doesn't give a distance that makes any sense. If it were WAY off, I wouldn't have expected on of the lens threads to start right at the infinity mark (?!?)

I'm disturbed (more than normal) by all three cameras I've tried doing this with, so I wonder if it's me (my eyes are bugged out too far from stress?).

Maybe I'll try this with a working SLR to make sure it isn't operator error.

I personally need a loupe to check this, but don't think the actual position of the loupe should affect the focal point, just the degree of enlargement and distortion and be fumbly to hold at the same time as rotating the lens cell. At least I put the camera on a tripod.

Any clarification available?