Well the manufacturer recommended only 12 SQ" and I went with 13.125" plus I rounded the port inside and out, and that gives me a bit better flow so functionally I got a bit more from that too. The customer is happy and that's all that matters, he will likely keep this setup for some time. This is not an AQ sub either its the FOsgate P2 dual 4 Ohm 250 RMS baby sub....1.9 cubes = 24sq in of port needed going by your math. Aq recomends 16/cube. Your quite a bit under.
And the Aq's like the high tune, they sound amazing @ 40hz