I say a minimum of 12 inches. Thats gonna put your fixtures about 8 inches off the water. That would be the minimum I say. Any closer you will get a dark spot in the center from the light being focused. I believe they recommend them almost twice that though.
i would build one 20in tall but drop the top section about 3in so theres a trim lip on the hood. this way you can store whatever you might like on top like brushes, dry food or whatever.
My canopy is only twelve inches. I'm planning on building another one 16 inches when I buy my new reflectors. Although Anleys is around the same height as mine now and it seems to light the tank pretty good.