Since I don’t own one of the vane type units yet, I couldn't say for sure, but can only surmise that as with anything else, a single CFM outlet reading alone won’t give you the full picture without taking into account other data like blower speed and duct volume capacity, etc.. to correlate.All the stuff they use to measure airflow with makes my head swim.
But the vane type with cfm reading, you would need to know the register or grill ka factor to calculate air flow, NO?
resticting the airflow to fit the vane will increase velocity and give a false reading.If you use a vane type.
You can make a cardboard box to cover the register. Then cut a hole in it for your vane(same size). Then you can ccalculate CFM based on the FPM and free area of the hole for your vanemanometer.