I am renovating my house and demoed existing sleeve cheapo pre-fab fireplace. Will update framing to create an alcove for a NM X33 that I purchased. Everything can dimension out properly 60' alcove height and 48" width, BUT I notice another dimension on spec sheet-see attachment-it's the diagonal line from front knob to wall "H" at 28"...WHAT IS THIS? Could it be amount front has to be inset from surrounding wall??? Also, plan on putting in slate floor, but can drywall line alcove walls and ceiling or is ceramic or stone material a safer bet??? Rest assured, I WILL hire licensed and experienced hearth co. to do install with HT pipe in chimney. THANKS!