My 20 year experience with a 2.8f leads me to the following suggestions:
Originally Posted by hmzimelka
Many 120 rollfilm cameras don't always hold the film flat in the middle, but the edges are held in place. That slight bulge in the middle makes the center focus farther away than the edges. This is probably what you are experiencing.
Indeed, a shim on the front cell will change curvature by making the perimeter of the field focus farther away and having not much effect on the rays near the center of the optical path. However, without using the Rolleiflex glass plate accessory to hold the film flat, your results of testing lens spacing will be subject to errors.
I'd just put the shim back where you found it.