DIY Carbon Filter + PC Case Fan. How Many CFM?

My grow box is 2.5x2.5x6.5 (40 ft3)

Using CFLs

Followed instructions to make my own cf from here https://www.rollitup.org/do-yourself/101248-best-diy-ez-walmart-carbon.html

Its inside the box and a PC case fan is doing the exhausting (sucking air) through it.

Well that is the concept at least, but I realized shortly after that my 57cfm case fan is not gonna be enough juice.

Ive seen on newegg they got case fans up to 250cfm and am more than likely gonna upgrade.

I couldnt find an answer in the DIY cf thread about how much cfm is needed to suck air through. Too much cfm and the charcoal may run out much quicker. What do you guys think about a 250cfm case fan sucking air through that cf in an environment like mine.

Thx

Your exhaust fan should be equal to intake, or slightly less, as you don't want to have less air circulating in the box. That's all I figured out for my PC grow case, I couldn't tell you a specific number though.