1 Mill = the angular height or width 1 foot at 1000 feet, or any other 1 unit at 1000 units.
So if a ship is 50 feet high and measures 3 mills high, simply take height / mills I.E. 50 / 3 = 16.6 and multiply by a thousand , 16.6 * 1000 = 16,600 feet away.
HiTech
"This new math doth amaze me! Tell me again how a swallow can carry a coconut!" Oh, and don't forget to divide by 3 to convert feet to yards. I still like the idea of recreating the system used by actual naval guns on ships. See:
http://mathscinotes.com/2013/08/battleship-rangefinders-and-geometry/ . I suspect Mr Fork got his USS Idaho pic from the same article. It would free us math-lazy types from having to grab a calculator while we're in the middle of a gun dual. Hey, "Give a mouse a cookie..."

Barring that, could we get the heights of all AH ships currently in-game, as suggested above? Oh, and another suggestion might be a clipboard app to do the math; select ship-type, input the mills, and out pops the range to target in yards. Hey, I can ask, right?
Thanks again, Hitech, for taking the time to interact with us and answer our endless series of questions. I really do appreciate it.