How to Determine Necessary CFM'S?

hello, i have a 40x40x70 grow tent, i need to determine the correct size fan, i know bigger can be better, but i am on a tight budget. need to determine the CFM necessary to completely purge the tent.
going to use a homemade carbon filter, made from a shop vac filter, and some activated charcoal. it will be attached to the exhaust end of the ventilation tubing.
i am using this for odor and temp control. any help would be appreciated.
i know there is a mathematical formula to figure this out, id like to know that so i can figure this out on my own next time. i dont know where to look.

if you know any good places to search for them as well, i know ebay of course, but i dont know the general price, i guess i can compare to htg supply, thank you in advance, and plus rep for all posts

Multiply to get your cu/ft. 3.4 x 3.4 x 5.10 = 59 sq/ft. You want to exchange your air in your grow tent roughly 5x per minute. 59 x 5 = 295 CFM is the fan you are looking for. You won't be able to use a 4" fan, so I would go with a 6" inline fan. You can always use a speed controller to reduce your CFM's.