[AllanJ]

For better rain shedding, have two diagonally opposite corners a little higher and the other two corners a little lower.

Most shade sails have two cables going to each corner support pole. To better prevent the corners from being torn apart. put a small metal ring or knotted rope loop around the two cables just at each corner of the sail. Add additional cables or ropes around the perimeter of the sail to keep the rings against the respective sail corners and the two cables entering each corner juxtaposed

Don't try to get every bit of sag out of the sail. As you try to do that, the amount of additional tension needed goes up exponentially and greatly increases the chance of tearing of the sail in the wind.