Main sheet and Traveler
The two most important guides for trimming a mainsail upwind are to keep the boom on the center line of the boat, and the top batten (three batten main) or the second batten down (four batten main) parallel to the boom. At this point, the leech telltale at the appropriate batten should flow. If it doesn’t, you should twist the sail off more by easing the sheet and pulling the traveler further to windward.
Leach tension
More leech tension closes the leech which allows the boat to point higher. But when you trim past the point of stall, where the top telltale (three batten main) or second telltale down (four batten main) stops flying, the boat will start to slow down. As the boat begins to be overpowered, the traveler should be eased down to reduce weather helm and keep the boat at less than 25 degrees of heel. As the traveler is eased, you will begin to develop back-wind. This is not a problem. In puffy wind, the traveler should be played aggressively. Choppy water usually requires a little more twist in the leech than flat water so you should pull the traveler higher and ease the sheet.