well   4 fans for hashboards

2 fans for psu.

the hash board fans  move at least 200 cfm each

but they are pointed trough the hashboards

so it is not 4 x 200 = 800

more like 2 x 225 = 450

tack on 50 for the psu fans

you get to 500 cfm

I feel that  500 cfm is pretty close to the top number they can do

Why not do
 a 500 cfm
 a 550 cfm
 a 600 cfm

set of calculations

it will give you 3 box sizes to choose from.