I've done about 7 or 8 aero boxes and I try to go with the most port area that I can fit. I've done everywhere from 7.5-12.5sqin per cube without any issues. I'm just going off of my experience with aeros, but I'd try to shoot for around 10sqin per cube. Aeros are a lot more efficient than slot ports so you don't need as much port area.
Sent from my Motorola 2950 Special Edition 3w.