atlas 15 dimensions

So as this being my first DIY project I really need some input because I am not completely sure of what I am doing. I bought a AA atlas 15" for HT use and I am now trying to build a ported box so it can hopefully get in the 18-20hz area. I will b using the 272 watt parts express plate amp unless someone else has other recommendations. I will also be using to 4" flared ports but I am not too sure about how long they should be. I have been using winISD some but it is confusing sometimes. (for example when it asks how many liters the box is or needs to be how do I decide?). Anyway with this info I hope someone can help me with a configuration of some sort.
 
Yes ports take away volume that you need to take into account. Usually it's not going to change your plan much.

Port length will depend on the internal volume you're aiming for. A bigger box will need a shorter port to achieve the same tuning.

I have been using winISD some but it is confusing sometimes. (for example when it asks how many liters the box is or needs to be how do I decide?).

Usually you aim to have a flat frequency response curve, you can allow a shallow rolloff as the frequency decrease because the room gain will compensate for this.

If you look at the red curve on this graph, you'll see that they allow a shallow rolloff to take into account the room gain :

http://www.ascendantaudio.com/archive/Atlas 15 ported low Q graph.htm

Forget anything above 100 Hz because you're building a subwoofer.

The volume of the box will also be limited by how big of a box you can tolerate hehe!