Going to have a 16x16 firebox door. Firebox will be 18x18x16 (Cinderblock lined with fire brick). Thinking of making air inlets/dampner with circle steel plates. One at the top and one at the bottom of the door. How many and what size should the holes be? Is there a general rule of thumb that your inlnet/damper size should be x amount of the size of your firebox?