A Blackman is more efficient @ speeds < 14-15 knots with the tabs all the way down, but after that the boat wants to get higher out of the water so you need to start backing off the tabs; by the time you hit 20-22 you should have a minimal amount of trim tab. But the cardinal rule is always leave your tabs up higher in a following sea so the bow doesn't bite in at an angle and suddenly turn to one side, very dangerous.