Firebox Sizing:

X	Width of the firebox opening	DF	Distance between the firebox and the floor
Y	Height of the firebox opening	RHH	Height of raised hearth (optional)
DL	Distance to nearest obstruction or edge left of the firebox	RHW	Width of raised hearth (optional)
DR	Distance to nearest obstruction or edge right of the firebox	RHD	Depth of raised hearth (optional)