It can't be expressed in feet, because I don't know how high the ceiling is. So instead, what you do is start at the ear height from the main viewing location, then locate the Atmos speakers in the ceiling about 25 degrees forward from that point. It doesn't have to be perfect, just close. If...
I’m guessing there’s no room behind the seating position, so I’d put them overhead at about 25-30 degrees forward. I know suggestions are to be further forward, but real world experience is showing that’s too far.