I'm looking for a methodology to easily measure/evaluate Z positioning accuracy, using equipment on hand or easily obtainable such as a high-precision digital caliper. In particular I want to be able to evaluate whether steps (actual motor steps, or some other chosen unit of increment) are uniformly the nominal step height, or whether some are larger than others, and if so whether the irregularity is reproducible. Has anyone worked out a way to attach a caliper or equivalent measurement device sufficiently rigidly to both a fixed point and a moving-in-Z point (e.g. the bed and the gantry on a typical cartesian machine) that the magnitude of individual steps can be read off? Or maybe a high precision laser range finder/interferometer solution is more appropriate, but I'm not sure about cost/availability.
I ask this in the context of recurring concerns about a common overconstained Z axis design (Ender 3 and nearly everything else with V rollers), but this question is intended to be about measuring not mitigations/fixes.
I'd also be interested in opinions on the necessary measurement precision for this measurement to be meaningful. Almost surely errors as small as 5% of the layer height will lead to some visual surface inconsistency, XY dimensional accuracy problems, and weak points for part to break at, which you'd need 10 micro resolution for 0.2 mm layers to see, but I suspect the reality is much worse for lots of printers and even 20 micron resolution or worse might aid in making serious improvements.