I will take a contrary position and say that a flow hood is the most important tool. Why? We all know houses are leaky, it's basically a given - blower doors are great but they tell us what we already know.
HVAC systems are the energy pigs. Checking airflow at the supplies can reveal all kinds of issues and is a critical part of obtaining real comfort at the end of a project.
We know we need to air seal virtually every house we work on. HVAC performance is a moving target and is not as easy to predict - this requires measurement , that why I vote for a flow hood.
Find a plumber who carries a pressure gauge and chances are you've found a good plumber.