The manuals are a good starting point but they are not 100% accurate. A lot of us with atmos systems can help you navigate that. As
@witchdoctor posted its all about the angles and getting those right.
P.S. I'm a little jealous because exposed joists means you can mount bookshelves with brackets at the perfect angles angled correctly to the listening area. Most of us have to work with in ceiling which is doable but not as optimal.
I had to go with Focal to get the angles right and great sound up top and those weren't cheap.