You are correct. But once you know what you have, you can then do the SP loss calculations for the duct work, fittings, et al. and compare the results to the fan curve. My fan curve is a two point (SP @0CFM, and CFM @0SP), and the the trash can separator SP loss is guessed for now (actually based on what I have for the 4" DC). Given that, the math says I am good with margin.
All I have to do now is figure out how to connect the DC to the plumbing.