Ive been having some issues on how to determine face velocity for washable metal filters.

I.E.

Im working with 1200 CFM and my duct sizes are 24"x24", therefore my veolcity would be 300 FPM ( before crossing the filter).
Those 300 fpm would be passing thru the filter and the filter would be generating a certain amount of pressure loss ( determined in Inwg)

How would I determine face velocity? I believe that once the air crossess the filter, the face velocity would not be 300 fpm. Im assuming there has to be some type of an "effective area" or some sort for the filters in order to determine face velocity.

Thanks in advance.