in lights 1 Watt is equal to 3.423 BTU/hr. so 4 1000W lights is equal to 13,692 BTU/hr. To keep it simple and on the safe side. Take your BTU/hr and divide by 25. which will give you 547cfm. It can get way more technical and really if you do it right you dont even need that much. But that will keep you cool.