how to figure tuning of my box?

I can't tell you how to tune it, but all I CAN say is that I'm jealous!  Sounds your subs are going to shatter some bones!

When I last used a ported Q-Logic box, I just called Crutchfield where I got the box, and got their answer.  Then I called Kenwood and asked them since they were Kenwood subs.  Both Kenwood & Crutchfield has the same answer for how long to cut the port down to.

This time I went with a sealed box, so I won't be worrying about ports.

Man, a pair of 15's?  I envy you. 

There was once a dude in a Suzuki Samorai that used to drive by the auto repair shop I worked at, which was right on a main street, and every time he was coming down the street, thing on our walls would shake.  When he got closer, things on our shop benches would vibrate, as would our tools boxes and our bones!  You could feel his bass from across the street.  Finally one day he pulled into the little mini mart across the street.  My boss said "Go find out what he has for subs!", so I did.  I almost wet myself when he showed me!  He had 6 (yes, SIX) of those huge Cerwin-Vega 5000-watt 15-inch Stroker subs, with each one being driven by a seperate Precision Power A-1200 amp!!!  All that in the back of a Samurai.  That didn't include all the MB Quarts he had elsewhere.  Man, I can feel the bass just thinking about how hard his pounded.

Good luck with your port tuning job!