I’m pretty sure I’ve got some unevenness too, but it’s tiny. I printed a 0.05mm single layer tests, and still had some slight problems. The edges print well, but there’s a thin spot near the front-right corner. It’s thinner, but I’m able to print a single complete 0.05mm thick layer. Since the calibration adjustment is only 0.05mm, I think I’ve done all I can.
I’ve used a straightedge and flashlight to see if I could find any unevenness, but the surface texture of the bed makes it difficult to test. I’ll redo the test if I ever peel the sticker off. I don’t have the machining tools to measure to a thousandths of an inch though. 0.05mm is 0.002", so we’re talking about fairly fine machining tolerances.
I like your test better than a solid layer though. The solid layer takes a while to print, and I don’t think it adds anything. At least, your test will cover all of the spots I know about on my bed.
I definitely recommend printing something like this after doing a paper calibration. I have a much better print now, for the cost of a few print-and-adjust cycles.