I was thinking about getting around that feast or famine scenario by encoding the user location as a geohash. And then chop off one char of precision from the end of the hash string on successive DB queries until a large enough group of people were included. Nice and easy to store and query the location as a plain string in the DB, too. Although - I was trying to figure out what to do about Greenwich near me, as that's where the most significant bit alters because of the prime meridian.