You must position your adjustable pole depending on the sail dimensions without omitting the distance between the pole and the corner of the sail necessary for the tension margin.
Be careful, this is all the more important when you implement your pole on an incline: the more you lower the attach position of the pole the more the tension distance will be reduced (because of the pole incline).
It is important to anticipate this tension reserve.
Shade sail size
It is always preferable to have an overly small sail that allows for optimal tension and a smooth sail rather than an overly large sail that will make the sail slack, unaesthetic, and with a pouch so more sensitive to wind.