SPÍRY
The rule change to permit longer battens for racing mains was a significant step forward for sail durability. Because the inboard batten ends now extend into the less-loaded area of the sail, the main flogs less and keeps its fast racing shape longer than ever before.
From a speed point of view, battens should support the roach (extra area in leech beyond a straight line from clew to headboard) without distorting sail shape. This means they must be very stiff at their outboard end. At the same time they must be quite flexible up forward, where they enter the more curved area of the sail.
Use stiffer battens (in the top one or two pockets) in heavy air to take curvature out of the leech. They are too soft if they allow a lot of curvature in the upper leech or fail to control the lower leech in strong winds (see below). Battens in the bottom two pockets should be very stiff (you can tape two together if necessary).
Use softer top battens in light air to avoid a hard edge at the forward end of the battens. The battens are too stiff (see below) if they create a kink running from the inboard end of one batten to the inboard ends of the other battens.
Leech cord
The leech cord should be set just tight enough to eliminate flutter. This may mean you get a slight hook in the leech. This is more unsightly than significant. It's better to reduce flutter, which destroys the sailcloth and tends to get worse with age.