So you’re working with 40 cubic feet of air, your CFM of 400 (max) will be 200 with a carbon filter, 100cfm @ 50%, 50cfm @ 25% etc. With your fan set to 2-3 and an open intake port, you should already be cycling more than your tents capacity for air while replacing it with air sucked in through your open port, so I don’t see a benefit in implementing an intake fan. That said, you could do a makeshift filter of some sort for that intake vent to keep pests and light from creeping in. Happy growing!