Heat rises so that's why vents high up are recommended, plus it keeps the drafts off the birds in the winter. If you can, put both below and above. I have vents on all sides of my coop. No matter what the weather I can adjust the inside temps and control the drafts. It's the best thing I did, but I didn't come up with it on my own....got all my information from other great BYC'ers!! I love visuals so here are lots of pics for you.
This is my run side. I have a slider door that I keep open in the hot summer months. It's right
at roost level so they they get the full benefit.
I have vents up high in the front that I don't close. Plus the windows in the front as well as the one on the side.
I have vents running all along the upper side of the back, above the roosts. You can see them also in the first pic, they're closed.
I made three different baffles so that I can open all or some of the vents. One opens into the service area so when
it's super windy (as it often is) I can let air in without it blasting the birds on the roosts.