Lane, it seems to me that it would be a lot simpler to just measure your flow instead of trying to guess at fan curves. Use a
DIY Pitot tube and your DIY manometer and find out what you actually have. The only "calibration" you need to do is calculating your air density and it sounds like you're already on top of that.
FWIW I'm using two 4" ducts to my table saw and I get enough lift on the topside to keep the duct clear. It's best to minimize the amount of flex in the path because it adds a lot of resistance unless it's pulled absolutely straight. Bends in that stuff cause more pressure drop than I ever expected.
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