DIY Homeowner here.

I have installed a ARU12RLFCD ducted mini split and it is working great in a dedicated home theater environment.

The unit is rated at 0.36 inWG and based on my runs I am currently at 0.24inWG when operating under full power (rarely do). At normal operating speeds in the room, the unit is rated at 250-300 cfm volume.

I want to plumb in fresh air into the unit and Fujitsu confirmed I can mix in fresh air on the return side in the mixing box up to 10% of unit capacity.

My supply and returns are 10" flex duct, so based on surface area a 3" duct is 10% of that but I do understand from fluid mechanics its not surface area but pressure drops that are equal, etc, etc.

Would any of you have any good practice recommendations for a fresh air intake design for my use without me running some complex equations to work this out?

My plan is to plumb in 3" PVC from the outside, with a ball valve to adjust the flow based on gut feel. The total length of the run of the intake would be 16 ft based on equivalent length of elbows, etc. Would 4" steel duct with a damper be better?

All I am looking for is some gut recommendations to solve this issue? My mini split is oversized for the room and have oodles of BTU's to spare. Room is sealed TIGHT based on professional home theater, but the amount of fresh air being sucked into room would be relatively tiny by comparison.

Appreciate the time and help.