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first thought was 0.2', when I counted pixels I got to 0.206
The index line usually is 0.1 or 0.05 unit wide, at least that was how I was taught to make markings when graduating "micrometers".
try to use that 0.1' as reference and interpolation is suddenly somewhat easier.
To make things confusing there are of course several standards, most also stating how long lines should be relative to marking width and number of subdivisions, how tall letters...
oddly most have that 0.1 or 0.05 index line width, one that I can recall has 0.25 and another 0.5 but they are rarely (read never) used.

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