From an alternate perspective, you need to know What the Sensible Load requirments are for day/night based on a selected Inside Air set-point.
CFM = Sensible Load / ( 1.08 * approx. 18)
ie. Room is 12 * 15 = 180 Sq. Feet which requires 2,000 BTU/Hr Daytime .:. 2,000 / 20 = approx. 100 CFM.
If you are targeting a night time set-point, the required load is going to be much less based on both decreased outdoor temperature
and NO load from glass.