It not only has to do with the Source Point but the direction of takeoff for the P-Trap. For Desig Line you want the direction in to be from the top of the P-Trap and not the leg. With the attacher arrow it would be reversed with the direction in from the leg. We use the P-Trap with Design Line and the attacher arrow so I have a button script set to "takeoff by other end" set as the default.