A misunderstanding of the old scientist/techie joke that a millihelen is the amount of beauty needed to launch one ship. Milli- is the international scientific prefix for 1/1000. Most international units are named after someone -- watt for power, newton for force, ampere for current, and so on. Scientists, techs, and gears who work with those prefixes all the time often attach them to all sorts of made-up units. So a megahelen would be the beauty of a face that launched a billion ships (one million times one thousand) and a nanohelen would be the face that launched one millionth of a ship (one billionth times one thousand). Millihelen worked better a few decades ago when newspapers and TV news often carried stories about ship launchings, because usually the woman christening the ship launch was a First Lady, the Queen, or some senator or lord's wife (what nowadays we call a first wife, the one before the trophy wife), and thus a lizardy old bag that was only sort of recognizably female.
"That one's a mini-Helen."
"Unless she's a real cute midget, you mean she's got about a millihelen. Go back to chemistry class and stop getting your science from Star Trek reruns!"
A face that launched 1 ship.
Nice kid, but kinda plain; a mini-Helen.