Thanks Ed! I didn't realize until I read your message that this 85mm film dimension was the diameter absolutely needed by the film. But of course, the diagonal is also the diameter...
However, my previous question was also about "what is a safe margin between the diameter absolutely needed by the film (or diagonal) and the diameter of the image produced by the hole (or coverage)": 1cm, 2cm, 4cm extra on each side???
So, indeed, I saw that the coverage would be 163.2 and would probably be sufficient taking 120 film. But I was more wondering about the quality of the image at the edges of the circle (coverage) and to what extend the quality is degrading (it seemed to be a problem with my 45mm tentative design where the film was barely covered by the pinhole image).
Actually, I'm just wondering how much I can safely decrease the focal length and still ensure a good image quality including in the corners.
Sorry if I wasn't that clear in my previous message (I hope this one helps!).