I am seeking advice on installing a cold air return. I am finishing a large open basement space. The floor is roughed in for radiant heat. The HVAC will serve both cooling and supplement the in-floor heat. I am tapping the existing trunks for cold air and supply. Is the formula really as technical as some make it sound? I am putting in two 6" supplies in the ceiling. Can I use 6" pipe for my cold air return? Do I need two?? Must I go to the floor?