I think you may be starting this from the wrong end.
First what are you trying do with the air?
Is this a supply duct or an exhaust duct?
Normal practice is to design a system to satisfy the design criteria then work out the external Resistance and select a fan to match both the volume flow against your calculated system resistance.
Give us some more information on your intended use, duct layout, numbers of take offs from the duct etc and I am sure we can help you.