There is in fact a complicated formula to figuring it out, I didn't care to learn it. The short answer it would seem is over 400cfm will do the trick for a 600 watt. So that's a 6" vortex style fan with probably as much as 100' of duct. A quick search revealed the following: 6" Fan - 79$ I believe I will order something along this line. Be careful that you don't get a bad 6" fan. Some of the ones I see listed on ebay range anywhere from 250 cfm to 450. So make sure you get one that goes at least 400cfm I recon.