Short answer- follow the dolby guidelines.
Long answer- Post dimensions of your room with furniture intended. In order for ATMOS to work correctly many factors must be considered.
The first question should be "how tall is my seating seatback?" The minimum height of the tweeter should be so that no listener is shadowed by either the seatback or an adjacent listener.
The second question is "What is the distance from the surrounds to the closest listener?" If some are seated very close, cheating the surrounds a little higher will reduce hotspotting and give more even coverage to the seating area.
The third question is "What is the ceiling height?" If the ceiling height is proportionally low to wall height, and you have cheated your surrounds higher for the above questions, you may find angled ceiling speakers are a good choice. This way you can avoid having to place ceiling speakers more narrow than the dolby layout and reduce hot spotting in lower ceiling designs.
There are a lot of factors to consider. To do it right you really should either research a lot or start a build thread and get help along the way or hire a professional to help.
Not the geek squad at BB either....
