It's in a document I pulled of the Dolby site dated July 2017 called:
If the chosen overhead speakers have a wide dispersion pattern (approximately 45 degrees from the acoustical reference axis over the audio band from 100 Hz to 10 kHz or wider), then speakers may be mounted facing directly downward. For speakers with narrower dispersion patterns, those with aimable or angled elements should be angled toward the primary listening position.
The overhead speakers should be at a height (shown as H3 in Figure 2) between two and three times the vertical position of the listener-level speakers. The angle of elevation from the listening position to the left top front/right top front and left top rear/right top rear overhead speakers in a 7.1.4 reference layout should be 45 degrees. This may be adjusted between 30 and 55 degrees if needed, as shown in Figure 2. Figures 3 and 4 show the preferred locations of the four overhead speakers as seen from above. The horizontal width should be about the same as the horizontal separation of left and right speakers placed at ±30 degrees. If this guidance is followed, the overhead side-to-side separation should be 0.5 to 0.7 of the width of the overall layout, depending on the distance to the screen and the front three speakers, relative to the surrounds. It is best to keep the overhead arrangement centered, front to back, over the listening area, even if the front speakers and screen are at a greater distance than the surround speakers."
It is 1.3MB so too large to attach, perhaps Google may help.