Reduce the load before chasing a rating
Duct leakage matters most when ducts pass through attics, crawlspaces, garages or other spaces outside the conditioned envelope. Conditioned supply air lost there does not reach the intended rooms.
Return leakage can pull hot, cold, humid or dusty air into the system from unintended locations, increasing the load on the equipment.
Duct sealing should target actual leakage and accessible defects rather than simply coating every visible joint without understanding the system.