Five single-thread thinwall boxes scattered across the platform had an average height of 2.99 mm, with a range of +0.04 mm, -0.06 mm:
The wall widths work out to 0.39 mm, with a range of +0.2 mm, -0.01 mm.
Close enough, given that I can’t recall the last time I tweaked the platform height. I update the filament diameter setting in Slic3r every now & again as the printer gradually works through the spool, but, with one exception, this cyan PETG has been quite consistent and my tweaks didn’t really amount to much.
Frankly, given that any of the measurements may be off by ±0.02, the best I can hope for is an overall warm fuzzy feeling. When the printed results stop looking good, these results will (probably) provide some indication of whatever just changed.
The raw measurement data, such as it is: