After building a bunch of stuff (about which more later), the Skirt extrusions showed a consistent tilt of about 0.1 mm downward to the right and the Skirt along the that side was consistently around 0.42 mm thick. So I cranked the right side down by 0.10 mm (just over 1/6 turn) and changed the G92 setting to Z1.50 to move the plate up by 0.05 mm.
Three test extrusions in quick succession gave these results (in multiples of 0.01 mm). I used the plates in reverse numerical sequence due to an accident of history:
It just won’t get any better than that!