We opted to install the front pair of mushroom vents to the sides, forward of the lower hinges, reasoning that we would probably have some pillows or something blocking the forward vents if left in the original location. The side panel placement also offers a more consistent (less curved) surface to drill and install, and is still about head height for sleeping. The measurement is 14″ between the upper piece of the lower hinge to the seam, and the center of the vent sits exactly at the 7″ mark.
We did try adding the excess foam to the mushroom top and removed the rain baffle as stated in the manual, but were unable to get the foam to contact the vent when the top screwed down. Eventually we just cut 5-6 threads off the screw and it fit. Cutting the screw also allows it to screw almost to the shell, reducing the chances of wind resistance and shearing while driving.