Update 7/14/14: I’ve posted a vertical loudspeaker aiming calculator which is a better starting point than this in most cases
One of the things on which I’ve been working of late is a formula to find the ideal speaker coverage angle for a seating area. It has involved a lot of trigonometry and double checking in MAPP, but I’ve found something which seems to work pretty well for single speaker applications and fits conveniently into an excel calculator: