6th Order Bandpass Calculations

I'm still struggling to understand 6th bandpass calculations. To convert a 4th order to a 6th order I simply put a port in the sealed enclosure, right?

But in order to calculate the port size do I treat the sealed portion of the cabinet as a separate enclosure?

e.g. 520 * 340 * 100 = 9 litres gross. A 55 * 170 port should offer me around 65Hz, right?

I'm unsure as to one enclosure's affect on the other.