Ratio analysis is a fantastic tool, used in the hard sciences all the time. My thesis, for instance. It allows to you reduce the uncertainty in your final answer by having potential mistakes "cancel" in the ratio. It is just a matter of scaling from a known solution (stock plenum airflow) to a new solution (your new-design plenum) using a factor you calculate. If you can't get the airflow for the new plenum directly, maybe you can calculate the scaling factor and get the stock plenum airflow and find the same answer indirectly.
