RE: ". . . can't find) anything in 621.1 which says that unoccupied ventilation should be zero."

I think this requirement comes from the mandatory provisions of 90.1.

6.4.3.4.3 Shutoff Damper Controls. Both outdoor
air supply and exhaust systems shall be equipped with motorized
dampers that will automatically shut when the systems or
spaces served are not in use. Ventilation outdoor air dampers
shall be capable of automatically shutting off during preoccupancy
building warm-up, cool down, and setback, except
when ventilation reduces energy costs (e.g., night purge) or
when ventilation must be supplied to meet code requirements.

Thoughts??

Reba