Depending on how deep the box is, your port will only reach so far. For example, your port may only need to be 36" long, but if your box is 48" long, you can extend your port until its at the correct point from the rear wall, and then increase your port area until you're back to the original tuning you are aiming for. I like to use this if I'm unsure on how much port area to use. Unless you're doing port wars or something, more port area is usually better. Why only have 12 sq. in of port per cube when you can have 16 or even 18?
