Luiz, the best would be to keep the camera as it is, hoping the lens to be the original one (after all these years...). But the most convenient for me is to set it at the Leica standard (28.80mm loaded). As you wrote, it raise another issue: are new lenses going to be on the right position once screwed? If I remember well, I have only one camera with such real trouble.
For the flange measure, I have a depth gauge which is perfect for that task. When light is correctly oriented, it is fairly easy to see when the gauge tip touches the film surface. I have to make several measurements with various orientations of the gauge base to check that the mont ring is parallel to the film plane. This is very time consuming!


