How to split a single fan's pressure evenly between unequal sized areas

go on amazon and buy a velometer for about 60 bucks. Its a tool we use to get the cfm of air moving from a fan or vent. you can use dampers that would be the easiest way without doing a ton of calculations. The unfortunate part is people dont understand that duct is rated and it doesnt matter what size fan you use you can only get som much air thru a duct depending on size and shape. A six inch round duct with no bends you can put about 140 cfm through once you start adding bends or y splitters you reduce what the duct will move. When you use a larger fan with the duct you get a higher static pressure which will cause turbulence and slow the air movement down. you can end up making a 140 cfm duct reduced down to 80 because the static is to great.

measuring the airflow is the only true way. If you have enough straight run for your fan and equal duct runs you will get equal airflow. The trick is....equal in pressure drop. If you have a filter on one end that's a huge pressure drop so you artificially have to make up that drop using a damper to shut off the air in the other duct. One problem is it's going to be hard to get perfect. With both rooms sealed perfectly ideally I guess you want your flowering inside room slightly negative to the big veg exterior room? Do you want to account for a door open or allow for air to leak out of the flower room when the door is open? Depending which option you want determine how much air difference you need I would guess 50-75cfm for the open door and 15-20cfm closed but that's a guess personally I would go more just to be sure. Do the rooms have separate intakes? Or are you pulling air through the big room to get to the small one? You could make a homemade manometer and read the pressure difference between the two rooms. Might be cheap to buy one and no power