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Hank - Bless Him - wrote the books on ACCA cooling calculations, duct design, etc. Amazon sells this book for a good bit less than ACCA.
Buy it and learn the "Standard of Care" for flexible duct, fitting equivalent lengths ("EL") and the multipliers to be applied for flexible duct workmanship; there're also graphs for flexible duct and lined or fibrous glass friction (NO - You can't use a metal duct Ductulator to size those!).
This manual is a must for contractors who design and install residential HVAC systems. I worked with a duct design firm when I built my house and referred to this manual. Unfortunately, most contractors don't use software (or even long hand calculations) to design duct systems...they estimate based upon past installations and by using square footage measurements only. While this is faster, it usually means your HVAC system is oversized or undersized and will not provide the expected comfort under a wide variety of conditions. Extra time spent in properly designing your HVAC system will reward the occupants with a more confortable living / working space.