Oh also, being able to tell how things visually change over distance is useful too. I use these less and less, but generally looking at a person at 100 meters you can make out a face. At 200 you can't see a face but see a head, shoulders and arms. at 300 you start to not be able to make out arms so well, and at 400 a head blends into the shape of the body. There's like a stack of different things you can use to create you're own references as well. Cities are great because generally if you can look down a long street you can see lots of things and people and start picking the differences in what you can characterize over a set of blocks.

Index fingers vary wildly in the length though, so how do you account for that?

Even taking some idea of an "average" hand is going to assume a lot when you start considering the differences between the sexes.

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.