HELP WITH SUNDOWN SA18 PORT

So I'm upgrading from 2 skar ddx 10s to a sundown sa v2 18. Obviously a big upgrade. Im looking for help with the port. Attached is a box design that's close to port/volume specs. Sundown suggest 35 hz and 68 sq inches. How do I get that closer. My mobile calculator only visualizes 1 port wall and Im having trouble with a port wall 2. TIA.