He tabbed over to the constraints. This was where the art happened. He had two choices: size by or size by Friction Loss .
Here is some information about duct sizer and McQuay: duct sizer mcquay
He hovered over the selection. He chose . He set the rate to 0.08 inches of water gauge per 100 feet. It was the "sweet spot"—quiet, efficient, standard commercial design. He tabbed over to the constraints