your 50 per cent part.
It's not ABSOLUTELY precise.
But three days on either side of a Full moon is 7 days (including the day itself)...add that to a New Moon, and 14 days each lunar cycle are within three days of a Full or New Moon.
I stand horribly guilty of saying 50% when I should have said 46.66666666666666666666666666666666666666% of a calendar month is within 3 days of a New or Full Moon.