Duct material now changes the answer. Galvanized, duct board and flexible duct each
carry their own ASHRAE roughness, and flex needs a full standard size more than metal
at the same airflow. Size a flex run as if it were metal and it always goes in over
its design friction rate, by up to double.

New Check tab for the duct that's already there: type in a round diameter or a width
and height and read back its velocity, its friction rate, and the airflow it will
actually carry, with a verdict against whichever limit binds first.

Also: a friction rate you type in now survives switching between supply and return.
Manual D designs a whole system to one rate, so the per-service defaults no longer
overwrite a number you worked out yourself.