I did, but did not mention it in the above as it seemed a bit out of scope. Used 1-2-3 blocks on the rails, with the calibration card as a feeler gauge.
I even double-checked when I saw the .31 difference: I put a caliper on the top of the X-axis rail and ran the depth-gauge part down each of the Y-axis rails. It was difficult to get a good reading but it was closer to .03mm than .3mm. I plan to bring a depth micrometer over from the shop and get a better reading.
On the plus side, my print succeeded, and it was one with a high failure rate so I alkready feel a lot better about this machine.
Quick update: I bolted a metal bar (weld steel, probably from the big-box store) on the back wall of the enclosure and stuck one of those cheap 6" magnetic-backed mirrors to it. Very handy for toolhead changes.
I also did the door: milled a 1/4" shoulder (a rabbet in the wood world) on two rectangular bars of aluminum, bolted them to the enclosure with the shoulder butted up against the frame. 1/4" mdf rides in the shoulder. I made an 11-1/2" square cutout and affixed one of those orange 12x12 acrylic laser shields using screen clips. Worked well enough, but I have it all taken apart for a re-do while the machine is configured for 3D printing. Will get pics later.