This is where I went when trying to find a solution for low ceilings and it worked out really well...
DOLBY ATMOS: DISPERSION REQUIREMENTS FOR CEILING SPEAKERS If you scroll down a little bit, it shows diagrams for lower ceilings with overhead speakers mounted on the ceiling vs in the ceiling to get proper angles.
Nyal Mellor is a pretty smart guy that did an amazing job with acoustical mapping and treatments for a home theater in my area that I had the privilege to demo a few years back. I mean the sound was SOOOO good. Granted, the home owner put in probably $100,000 $125,000 into the theater room itself and that was all DIY speakers and DIY acoustical treatments that were developed by Nyal to "keep costs down." So if he put in commercially available speakers and treatments for absorption and diffusion, or had contractors build out the space itself (outside of the 10 ft "pit" for the front of the theater when the house was built) it would have been even more expensive.
