When designing a basic style enclosure which I am assuming that is all you are doing here.
You need to take port/sub displacement into account to ensure you get the correct net volume. Your gross volume is much higher then what the driver actually sees.
The bigger choices to make when designing an "entry" level enclosure is what type of port will you use? Round or other(slot type of some sort)....
Once you figure that determine what your port area needs to be based on the sub used and the net volume. Once you get port area the length can be adjusted to achieve the tuning you are going after. Once you have those things figured out you can calculate the displacement of the port itself. Add the driver displacement (and any bracing or other obstructions), with the port displacement, and net volume and you get your gross volume.
This is a bit of a crude method however, for basic enclosures it will work just fine.