Does everyone know you can make crazy accurate measurements of layer height, line width, hole diameter, etc. with a (2D) scanner?
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Put your part on the scanner and scan what you want to measure at the highest resolution your scanner allows (that’s 1200dpi on my all-in-one but some dedicated ones are 2400 and interpolate up to 9600). On the image below I also scanned a ruler to the left (out of focus because it wasn’t pressed down on the glass).
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If you didn’t scan at around 9600 dpi, scale the image with a quality algorithm (Lanczos on IrfanView or default on Photoshop, etc.). I scaled mine up 1000% to 12,000dpi. If you don’t do this, the resolution won’t be high enough to accurately measure in the micron range.
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Set your units to mm in your graphics program, and measure away!
You can measure just one line/layer, but if parallel lines seem to visually be the same width, you can measure multiple lines and average them to get a more accurate measurement.
This is much more accurate than you can usually get with a micrometer and you can really see what’s going on. This really helps me dial-in my extruder settings, Z offsets, etc., as well as troubleshooting prints. If you make the image gray (which I did along with gamma adjustments to maximize contrast), you can even pretend you have a personal scanning electron microscope. 
