Hello - I am finishing my basement in the Chicago area and doing the new ductwork myself. I was wondering how to determine the size and location of the returns. Total finished area is 1200 sq. ft. With one bedroom (110 sq. ft.) and one bathroom (110 sq. ft.). I installed 5 supplies w/ 6" pipe. One in the bathroom, one in the bedroom, and three in the main living area. Admittedly...I based the size/location of the supplies on other basements of similar size in my area. I am looking at two returns to install, one in the bedroom ( no undercuts allowed in my area) and one in the main room. Problem is, not sure how to calculate the size? I've read a bit about using manual D, but not sure how I would do this without knowing much about the existing ductwork in the house (lengths, elbows etc...) because it is hidden behind walls. Also, I've read conflicting opinions on whether supplies in a basement should be located near the floor or ceiling?