The Skeinforge Outline plugin draws a rectangle around the first perimeter layer of an object. I use that single-width, single-layer extrusion to monitor the height of the nozzle above the build platform and the tilt of the plate. The Outline extrusion will either peel off separately or come off as the film peels away from the plate when I twist the object off.
These Outlines come from a variety of objects. The one in the lower left was a test case that I stopped after extruding only the Outline.
I measure the Outline along each edge; larger objects provide three data points along each side of the build platform.
The good part of this is that it reports the build platform’s behavior during an actual extrusion, so you can keep an eye on whether it’s drifting out of alignment. The aluminum plates present a sufficiently flat surface that any variations will be due to a non-level HBP or an off-calibration Z-axis.
These numbers from around a large Outline told me that I should tweak the Z axis height down by 0.1 mm to increase the first layer thickness back to about 0.33 mm. The lower-right corner was slightly thicker because the wiper hit the Thermal Core insulation.
Given those values, I can tweak the leveling screws to adjust the platform tilt. What I don’t have at this point is any long-term record of how consistent my hacked HBP will be. But at least I’ve got numbers!