you tune the port usually depends on the length, width and height, go to http://www.loudspeakers101.com/VentCal1.htm go to the bottom calculator click on square port and slot port and just type in what you got to work with it will show you the vent length
you have to get a box design program such as WINisd which is free. You type in box volume...desired port size...desired tuning and it spit out the needed port length. peace