For the sub box gurus?

Ok so I've been doing research on sub boxes and I have decided that I want to make my sealed box into a ported box. I had initially planned to make it ported but got lazy. Anyways I need help determining the port size and placement. The dimensions of the box are W= 30 H = 14 L = 15 and using 3/4 MDF so all internal dimensions are .75 less from all sides. The 12" sub sits as far left as the box permits and sits in there good. Ok so far so good but then I get to the port and I don't quite understand what this tuning frequency thing is or how it works. I am making the port a rectangle also if that helps anything. So if anyone would be so kind as to help me out with this problem. Oh and I also prefer the deep sound of b over sound quality that's why I'm going ported.Thanks in advance for the help.