well then...I guess I am right back where I started before I ever posted anything:confused
All of the duct work the air travels through is part of the info needed to tell what the FR should be.
Your return duct is carrying the air too. So it is part of the restriction to air flow, and has to be included.
Contractor locator map
How many times must one fix something before it is fixed?
http://www.acca.org/Files/?id=66 - some guidance in getting your ductwork system right in accordance to Manual D standards.