How to calculate FPM as related to Residential HVAC air handler
Your method of FPM X cross sectional area won’t come out right for a large trunk line, the problem is the airflow is faster in the middle and slower at the edges so usually a grid of measurements are taken and an average flow is created. I have a book that gives the recommended number of points based on the size of memory serves 3 points 2 on the sides and one in the middle averaged gets close with the big book. Measurements were not supposed to be taken the width of the opening from a turn, that’s silly because most openings are at a 90 to the duct within an inch or so. I just take the measurements wherever and it has always been close enough for residential.