A series of telltales along the leech of the main should flow evenly. If the top telltale stalls and curls around the back of the sail, the sail is trimmed too tightly. Another helpful indicator is to position yourself so you can sight up the sail, from below the boom, and line up the top batten with the boom. If the batten hooks to windward, the sail is trimmed in too hard. If the batten appears to fall off to leeward, the sail isn't trimmed in tight enough.
Draft refers to the curvature or fullness of a sail. The maximum draft should be positioned about half of the way aft in the sail. When you take tension on a sail, the draft moves in that direction: If you want to sail closer to the wind, trim the sail in to move the center of draft aft. For more speed, ease the mainsail out, which moves the draft forward.