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Women's fingers tend to be appreciably thinner, yes, given that this is infantry lore it's safe to say that it's calibrated to men in fighting trim (which implies no extra fat on the fingers).

There's still some variation but (the only reason I'm commenting) larger hands tend to be on longer arms, so this normalizes well enough for a rule of thumb.

Last point is that with a rangefinder one can calibrate this to one's own body, which is, after all, what gets used as a metric.

I think he's referring to finger thickness, not length. Index and middle fingers together supports this view, as well as the closed fist.

Yes, I am. Sorry I didn't describe that well. It's usually something you demonstrate in person.

It's not a perfect system, it's a estimate system. Generally it'll work generally well. Having a set of optics with a mils reticle really too, because then you can take the human element out of it.