This is from a previous post: The main sheet traveler has the primary function of controlling the point of trim, inboard or outboard, of where the main sheet block attaches to the boat. I find the following baseline settings to work as the starting place to fine tune the main sail. Wind 0- 5 set the main boom slightly to windward of the boat’s center
line. 5-10 set the main boom at boats center
line. 10-15 set the traveler to the full outboard position; this reduces heeling and allows the air to escape more freely from the leach area of the mainsail. 15-25 set the traveler ½ the distance between the centerline of the boat and the leeward aft corner of the boat. Letting the boom out any further will now bring into play the
boom vang This is my approach others may have different thoughts.
There is a good discussion on this subject in the forums under Traveler usage