Finally I have achieved comprehensive success with fixing and calibrating my Mamiya C330's focus !

I took the long and hard path when a few different decisions earlier on would have seen this solved months ago. Here is my advice for anybody attempting to fix/calibrate their TLR's focus.
  1. Use a focus test chart for close up checking. i.e. distances that have the chart fill at least 2/3 of the viewing frame, preferably much more.
  2. Ensure the test chart is 45degrees, but most importantly ensure the lines and paper chart are as close to vertical as possible using a small spirit level.
  3. When using the test chart ensure the camera's lenses are vertical using a spirit level.
  4. You will quickly see if the camera's lenses do not focus exactly on the test chart's centre line. Use this fact to align the viewing and taking lenses much more quickly than by trying to continually toggle the focus. (perhaps using a split screen focus screen would have helped here, but I didn't have one).
  5. When testing the focus at other distances, you have two reliable options
    1. Use a set of vernier callipers to compare the distance your lens plane extends from the camera's body when focusing through the view screen and then at the film plane. If you don't have a split screen focus then vernier callipers are SOOO much more repeatable compared to trying to focus on a flat image and remembering how in/out of focus the prior view was, OR
    2. Find something like a picket fence or a brick wall that runs away from you at an angle and place a marker next to one picket or vertical mortar line and do comparative focusing on that.
  6. When focus testing at infinity, use one of two methods
    1. A set of vernier callipers as above, OR
    2. The back sighting method (or a collimator if you have one!) to check/calibrate your camera. Fortunately in my case when it was in cal. close up, it was also in cal at infinity.
  7. When you have finished using a ground glass/perspex sheet+magic tape at the film plane, run through a roll of film. Take shots of the focus screen at close distances, then for any further objects up to infinity choose any object with good amount of detail/local contrast and take 3 shots. One in focus, one with the focus slightly too close, one with the focus slightly too far. Then using a loupe and a light box, you can compare the three negatives for sharpness.
  8. Know what tolerances you are aiming for. In my case I wanted at least 0.1mm repeatability since my Depth of Focus was only about 0.25mm. My callipers had a resolution of 0.01mm. When you focus on a line and measure the bellows extension with the callipers then re-focus your manual adjustment of the focus knob, you will not give better than a +/- 0.1mm repeatability.
In another post to this thread I will write about the problems I encountered and what I would have done earlier on to get a faster solution.

Here is a sample focus chart showing my success: