Help figuring out port length...calculate before port displacement or after

[46panel 10+ year member]

I know I need to account for sub displacement and I think I do for the port also but just wanting to make sure. I have 20.29 cubes before anything and then 19.54 after the subs. Using 4 8" ports using 19.54 the ports need to be 18" to be 34hz if I calculate the port displacement then it would be around 17.5 cubic feet and they would need to be 20" to be at 34 hz. Thanks and here is a quick little draw up I did ha //content.invisioncic.com/y282845/emoticons/naughty.gif.94359f346c0f1259df8038d60b41863e.gif excited!

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[46panel 10+ year member]

Ok yea its got to be after port displacement. It just makes it kind of hard to figure out because to see how much displacement the ports takes up you need to know the length of the ports and to know the length of the ports you need to know the cubic feet after all displacements. HMM frustrating

OR does the calculator just automatically figure out how much displacement when I type everything in.