ok here goes. I am from the north.When I worked on new installs, The boss always had the metal ductwork all premade, and a layout was waiting for us grunts to do the actual install, along with the service calls that may come in as well. The duct work and it's design was like another trade. That brings be to my question. I am on the Gulf coast, where return ducts dont exist, and anything other than duct board is unusual. The air handler is installed in a central location ,usually a hall closet. I have been asked to replace the system in a Katrina damaged building that had a 2 ton straight cool with 10kw strip heaters. I have the unit all lined up, and ready to start the task at hand. now the question. When I form the plenum above the air handler, with the area standard duct board, where do I find a formula for size in relation to CFM, in order to keep my air moving decent.all through the plenum to the last drop. I plan on 6 inch flex to the defusers off the plenum.all fairly straight andI figure I could just wing it, but them I would be no different that the next fellow who could use a stapler and tape thank.