In-ceiling speaker density

Hi everyone,

Would anyone be willing to tell me a theory behind how to design an in-ceiling speaker system for ambient music?  I imagine the right answer depends on things like
 - what are you trying to achieve (eg, in my case reasonable fidelity but also uniformity of volume as i walk through the room)
 - what's the ceiling height (in my case 9 feet)
 - at what height are your ears (in my case i'll be standing most of the time, so say 5.5ft off the ground)
 - what are the dispersion characteristics of the speakers (i haven't chosen speakers yet, and i can't find specifications of these data for most speakers)
 - probably a bunch of other things. 

thanks in advance for your help.  i have searched high and low...  and know that people have to deal with this all the time...  but i'm turning up very little.