RR: " So, if civilian night is when the sun is 6 degrees below the horizon, that means 24 minutes after the sun sunk below the horizon. Half an hour is near as dammit to that, perhaps this is the source of the half-hour rule."
Sorry, I don't buy that at all. Nothing in celestial mechanics is ever that simple. Maybe it works for you because you live on the equator, or another planet.
For some people who live in Finland, for example, the sun never reaches 6 degrees below the horizon for many weeks in summer. Come to think of it, maybe Finnish GA types like to fly around at midnight without bothering about night ratings.