The 4 inch fan is rated 190 CFM, but this 4inch fan I want to use just for intake fresh air. Then I was planning on using a 6 inch exhaust with 450 CFM, with my calculation I have 270 cubic feet of space , so 6x6=36x7.5' = 270 I WANT THE AIR TO CIRCULATE 6X'S AN HOUR SO I MULTIPLIDE 270X6=1620...