I'm about to install a new flange/pipe in my bathroom and have a question about installation. Right now I have a 3/4" plywood subfloor. I'm going to put 1/2 inch Wonderboard down, and then a layer of tile. At which layer of floor should the flange be installed? I was thinking that I would attach it to the Wonderboard layer, because going through the tile would quite difficult. Suggestions?