The thickness of hair varies from 0.06 to 0.1, so 0.1 is on the thickest side of the spectrum.
I’d need to see how you did the math on a noticing the curvature of the Earth across a football field. I’m curious because I setup guitars all the time and I can easily eyeball a 0.3mm gap and be really close when I confirm it with feeler gauges or a caliper.
Visualizing the thickness of hairs is difficult. Using a high quality caliper that can measure with 0.001mm accuracy, I found that 0.4mm is about the thickness of a standard envelop measured at the corner (so you’re measuring the thickness of the entire envelop including both sides and the flap).