400 cfm is the standard. There may be some exotic units that require a different value but 400 is the norm. http://www.acca.org/tech/manualj/ I’ve been a HVAC tech for 20 years. Manual J is the means by which a house is properly sized. It takes into consideration insulation values, sq ft, how many windows etc. Many are not sized by that method but to do it correctly that is the standard.