it's not just about CFM, it's more to do with the amount of static pressure it can build. You can have a 400CFM fan that is useless with a filter becaues it doesn't have "the balls"; whereas, another 400 CFM with a stronger suck/pull would work fine. Everyone tries to go cheap on ventilation and I think it's a very poor decision. Buy bigger than you need. That way, you don't have to buy again when you grow a bigger grow. Also, ventilation is ESSENTIAL. You can ruin a whole crop in one hot day without proper venting.