Try a TSI Velocicalc. I have one and I love it. This is a hot wire
annemometer with other options, dependong on the model. If you enter
the duct size, whether round or rectangular, and do the traverse, it
will give you actual CFM. It does all the math for you to eliminate
math mistakes. Mine also has pressure ports for hooking up a pitot
tube, good of there is dust in the duct when you test, still does the
math. Dust will ruin the thernal sensor over time, Mine also measures
differentioa bressure across coils etc. And it measures wet bulb, dew
point and relative humidity in a duct. Telescoping probe i 40" long,
so you will have to measure from both sides of the duct, unless you can
buy a 60 inch pitot tube. TSI do not sell pitot tubes but dwyer does.
Very versatile instrument, I love vaving mine. The model I have was
about $2100.00. Not cheap, but worth it.