Placement of Dolby Atmos ceiling speakers


I just got a Onkyo TX-NR838 receiver last month for $500 A $TEAL!
It has just about every new feature anyone could possibly ask for (HDMI 2.0, Dolby Atmos THX etc.).
At present I have it set up as a 5.1 system & it sounds fantastic driving my all Eggleston Works speakers.
For about $130 I can add a pair of the Onkyo SKH-410 Dolby Atmos speakers.
My main concern is that my family room has 15' cathedral ceilings & I am not sure where or how to place the Atmos speakers for optimum sound effect.
Anyone out there have the same situation and a solution?

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dolby website for atmos. Then tinker and adjust for sound you like. What else is there prob?