Direct reflection for speakers point.

I think I understand what you are asking: 

The point where you can see the speaker’s tweeter in the mirror should generally be near the center of the panel, both horizontally and vertically. The reflection is really an area rather than a precise point, so the panel should cover space on either side of the mark. Be sure to locate the reflection points for both speakers from the listening position.

Good advice above, as all frequencies do not spread the same. Also thicker is alway better as it absorbs a wider range of frequencies and will make the system sound less dark. I know that is counter intuitive.
 

What it is worth I have four 7” thick panels at each first reflection point.