The wiring is right. The 13 cu.ft. is the net volume, you add the displacement of the the sub, port and bracing to it. Contact CSS to find out the displacement of the sub (link at the top of the page), a port 8 inches in diameter 40.25 inches long is 1.1 cu. feet, and the bracing you'll have to figure out as it depends on how much you use.
Hey Mike ... since I am getting closer to finishing the theater now ... one last question regarding the SDX15 ... if I were to use a slot port for this 13 cu ft design ... what would the dimensions of the port need to be ( cu ft ? ) ? ... that way once I decide on the dimensions of the box I can modify the size of the port to the box
No, the surface area of the width and height has to remain the same. 4" X 10" = 40 square inches of surface area, 5" x 8" would be the same. The length doesn't change. The surface area of the port controls the air speed, the length of the port controls the tuning frequency.