Rule of thumb:
1 - wall mounted speakers project about 20-25'
2 - horns, while not sounding as good as standard wall mount speakers (almost no bass), project about 30-35'
3 - ceiling mounted speakers.......
3a - for ADEQUATE sound, place the speakers twice as far apart as the distance from the speaker TO THE FLOOR (10' ceiling = 20' spacing).
3b - for BETTER quality sound, place the speakers twice as far apart as the distance from the speaker TO THE EARS OF THE PERSON HEARING IT (10' ceiling, room has chairs, average person is sitting 3.5' above the floor: 6.5' to the ceiling = 13' spacing. 10' ceiling in a corridor. Average persons ears 5.5' above the floor: 4.5' to the ceiling = 9' spacing)
Having said this, by all means, talk to Valcom, they're a very good source. You could also talk to Bogen or Atlas.
Sam