put 6 mil plastic on the studs, run it down over the lip of the pan. Ideally, you shim the studs (or notch them) so the cbu can run down to within about 1/4" of the horizontal ledge in the pan. If you don't want to do that, go to the top of the flange. No need to silicon that junction since the plastic will protect the studs. Caulk the bottom of the tile to the pan instead of grouting and the corners. If you prefer not using grout, using an expansion trim like Rondec from www.schluter.com will work. That way, you could avoid caulk in the entire shower. Check out www.johnbridge.com for tiling questions. Note, the height of that flange and the size of the tile will make a difference. As long as more than 1/2 of the tile will be supported by the cbu, you don't have to run the cbu down, but if you are using small tiles, it becomes more important to run the cbu down so it has good support. You can use thinset under the tile on the flange to fill it in. Do NOT use mastic...use a thinset you mix up from the bagged powder.