MANY YEARS AGO, I paid good money out of my own pocket to learn how to design a duct system. I won’t say to whom I paid this money, because I got ripped off. What I “learned” was how to find .1 on a ductulator. You may have had a similar experience. And for the duration of this article, where I use .1, you can insert your magic number (.08, .06, this one on supply and that one on return, whatever…) instead.
That’s not duct design. And if you aren’t changing the blower characteristics at the end it’s not even blower design. It’s just half-ass design.