I'm a novice grower but I design odor control systems for many industries like plastic extrusion vents where the odors and VOC's are pretty high. Working out your carbon filtration is really just a matter of doing some math. Hopefully some of you will find this useful.
For carbon to work properly the smelly air needs a certain amount of "dwell time" in the carbon. I see a lot of people use fan systems that are too poweful and the air gets pulled too quickly through the carbon. The prices some of you are paying on amazon and other places is high also. Get in touch with these guys
http://www.generalcarbon.com/ and ask for a carbon pellet that is formulated for cannabis. Potassium permanganate is another odor absortion media works well. There are literally 100's of different carbon formulas for VOC and odor control.
To work out your cfm needed for 10 air changes per hour you multiply your fan systems cfm x 60 and divide by the volume of the space. For example...if a 4" cent' fan has a cfm rating of 187cfm and your space is 6' x 8' x 8' then 187x60=11,200/384 = 29.1 ach.
So..a 4" fan that does 187 cfm running on full speed with an line carbon filter is pulling too fast and the air is not getting adequate "dwell time"...but that same fan running at 1/2 or 1/3 speed is going to be much more effective.