We are creating a fireplace facing of stone/brick. We cannot find any information regarding the depth for the stone/brick in relation to the depth of the mantel. Our fireplace is a direct vent gas and it is flush into the wall and the floor, so we would be building the facing outward (fireplace itself would be inset). We want a deep, rhick mantel, preferably 12 inches deep, but want it to look proportionate to the facing depth. Any advise would be appreciated!!