eh, it's okay. whenever i do a design, i do 15-16 Inches per cube 90% of the time.
13 should be alright. if you wanted to be sure you had enough port area, go for at least 14.
and don't worry haha. all the headache will pay off once the box is built. use the calculator a few more times and you'll get the hang of it. it takes me about 10 minutes or so to get a design done with the calc