We're renovating our kitchen/family room with help from some contractors. Our brick fireplace is being drywalled, with a one-foot area of cement board bordering the firebox for glass tile. The drywallers are putting "high hat" as metal furring strips to anchor the cement board to the brick. They're asking want I plan to do on the "return" -- the thickness of the tile, cement board, and high-hat that will show on the side of the firebox opening. Uhm... I have no idea. Help please? I'm attaching a drawing. The "return" is in pink.