I am building a ported box for a old sony 10" sub rated at 150wrms/600wmax. The box is going to be around 1.3-1.5 cubic feet. My question is, what should i tune it to? The slot port is going to be 1.5"x12", i just have to decide the length. At 1.3 cubic feet, RE enclosure calculator recommend a port length of 25". does this sound right?