You lose the effective drive from the top of the sail through "tip vortex" anyway (regardless of the cut or shape) so the object of the shaping at the top of the sail is to reduce "drag" as much as possible, and the best (and only) way to do that is to have the top of the sail flat. The principle of the square head is not so much to add area but to "flick off" to leeward in the gusts thereby reducing heeling moments and increasing the ratio for forward movement, in so doing giving you more control of the directional stability of the boat. The weight of the top batten is not really important as long as it keeps the head of the sail flat. To make it "lay off", is controlled totally by the amount of luff tension that is applied to the sail. If it will not lay off regardless of how it is set up, then there is an incompatability in the cut of the sail with the mast, usually there is either to much luff round cut into the luff, or the mast section is too stiff for the luff round in the sail.