Change the tuning frequency of the cabinet, and it will give you back the length needed for that frequency, based on the size and number of ports you've entered.
To reverse engineer your box, take a stab at the tuning frequency, then compare it to your actual port length, and adjust as needed.
These are models. To find the actual tuning frequency of your cab, you pass signal through it and find the frequency where it holds the cone pretty much close to still.
That should be close to the model, but not always right on.
That Legend is fairly forgiving of varying box sizes and port tunings, is a good performer, and thus a good replacement for a lot of bass cabs.