A lot of it is down to experience, with in reason a yacht hull is designed to be sailed as upright as possible, or with a slight angle of heel, if your yacht is being blown flat all the time you are on a hiding to nothing, on the other hand if it drifts about upright you will get there slowly.
Watch other members of your club, you will see the ones regularly winning, watch and learn.
If a course is set up with a lot of reaching and running you will get away with more sail area than if a proper course is set where you have to beat to windward, this is where the tactics come in, can you maintain good windward speed and also keep up down wind. Shaking sails slow a boat down.
As you can see there isn't a hard and fast rule, the wind is never steady, its a law of averages.