I shoot macro with a Sinar P. Keep in mind that the focusing mechanisms don't have locks. They are very sturdy but I have noticed that if I point the camera straight down with a heavy lens on the front standard, some focus shift is possible. It happens slowly, while you are loading the film and making the shot.
One recommendation would be to use a shorter lens. At macro distances, even a modest decrease in focal length translates to a large savings of bellows draw.
For example at 1:1 a 250mm lens requires 500mm bellows draw. A 200mm lens requires only 400mm of draw, a savings of 100mm or 4 inches: enough to make the difference between comfort and... agony
Another suggestion is to shoot in horizontal orientation.
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