Imperious measurements

vashti linked to a US-authored guide to building a tin-can wireless antenna. The measurements are all in inches, which got me thinking.

How do you measure 1.21" in any meaningful manner? The rulers I have here are almost all marked out in thirty-secondths on the inches side, and whilst one has tenths for half its length, it doesn't go any finer than that. I don't even think you could fit another ten subdivisions into one of those and still be able to use it. Possibly with a special magnifying section, I suppose...

Best answer gets a cookie.