can someone help! I have T-shirt dryer Vastex X3-D 54" version, which has 8" duct and 550 CFM exhaust fan, it is connected to about 40 ft long ducts so I have another AC infinity exhaust fan on the other side, how do I know what speed to set on AC infinity fan?

can someone help! I have T-shirt dryer Vastex X3-D 54" version, which has 8" duct and 550 CFM exhaust fan, it is connected to about 40 ft long ducts so I have another AC infinity exhaust fan on the other side, how do I know what speed to set on AC infinity fan?