I think this video by the Dolby Institute is targeted at professionals but the principals translate to the home. You’ll notice that they don’t use the ceiling speakers pointing straight down but bookshelves that are angled toward the MLP. The other thing you’ll notice is that all of the bed channel speakers are at the same height (that includes the center channel). At 16:00 minute in the video make sure to watch the part about room tuning, very helpful as to what DSP can and cannot do:
" I feel like money spent on this type of help (room treatments and acoustics) will be more valuable than any piece of gear you will ever buy"