for a 400 watt cooltube you want a minimum of about 465cfm to properly cool the bulb. i'm not sure watt 465cfm converts to in metric, sorry. i had a 400hps cooltube with only 4inch duct booster and it wasn't so good but i upgraded to a 465 squirrellcage fan going by specs on the cooltube package and it was 100x better. with the 465cfm 400hps cooltube i could keep the temps in the closet the same as the ambient temp in my house at all times!