Well your ventilation system should be able to keep the tent within 5% RH of the room it's in. You wanna exchange the volume of the tent at least 2 times a minute, better 2.5 or 3. That fan should be enough CFM but probably not enough real world CFM because of the filter and duct restrictions or some issue with intake being inadequate. If restrictions can't be eliminated you may need to change up to a 6" setup to have a much better chance of keeping the humidity under control to prevent bud rot and powdery mildew. You may try pulling air without the filter to see if the humidity drops or disconnect any long external duct runs. Something is causing a massive reduction in the CFM.