Need a little advise before ordering fans.....

Would like to get a vote on size of fans I might be able to use before I buy these. I have a 2x4x7 tent with a 400 watt air-cooled hood. Now being that I have only 56cubic feet I figure a 4'' inline fan and 6' exhaust will suffice. I beleieve a rule of thumb is to exhaust all air out of the tent at least 20 times per ? If the intake is rated between 86 and 100 cfm's will that be fine for intake but what might he cfm's for the exhaust. Thanks.................

i'm sure any inline fan you get will suffice