The Bottom of Your Arc
In every round of golf there are no two lies that are exactly alike. The practice swing should determine where the bottom of your arc is for the terrain that you are standing on. If you notice that your arc bottoms out further back in your stance, play the ball back. If it bottoms farther forward then play the ball farther forward in your stance.
Knowing where your swing arc bottoms out will help prevent thin or fat shots. Keep this in mind every time you make a practice swing and watch your scores drop.