The easiest way to calculate room pressurization volumes and more like a rule-of-thumb method used by the industry is 0.25 cfm per ft2 of floor area for 39° to 50°F and 0.5 cfm per ft2 of floor area for 55°F and higher room temperature areas. Colder rooms require lower air pressurization volume as they more tightly built.
Added to the calculated air pressurization is the exhausted air volume from the room if any [ process system or, etc…]
Example,
1000 ft2 room floor area and 500 cfm exhausted by stacks [0.25 cfm/ft2 design for pressurization]
1000 ft2 x 0.25 cfm/ft2 = 250 cfm (for air pressurization volume )
500 cfm (exhausted by stacks)
Total: 250 cfm + 500 cfm = 750 cfm
Notes; the air pressurization volume must overcome all the opening, [cracks, conveyors, door infiltration], that can be calculated through ASHRAE formula, also must include the people fresh air intake of 25cfm/per person
The fan used for pressurization comes have external static pressure design in-wg [0, 0.5, ¾, 1, 1.25 in-wg etc..] and should overcome calculated friction losses through ducts, louvers etc..

Regards

Camille Zabbal