I need to balance the air flow in a system, preferably with dampers installed in the 6 inch round metal branch ducts. Some of these branches are up to 14 feet away from the trunkline. Where's the best place to put the dampers-closer to the trunkline or closer to each register boot? Either spot is accessable through the basement's drop ceiling.
Absolutely, positively, the best location is close to the main trunk. Your damper should be in the round pipe just after it comes off of the main trunkline. Your registers probably have opposed blade dampers already on them for some control at the outlet, but the airflow should be controlled as indicated above.