the top of the backswing
At the top of the backswing it is important that your club is parallel to the ground. If you swing further than this, then you are risking losing accuracy on your drives.
Here is some golf driving tips from Eric Jones: when doing a backswing, standing front of a glass window and look at your reflection at the top of the swing. You want to be in a position where you’re clubface is flat and parallel with the ground, this is the perfect position for your club.
Eric Jones is a former world long drive champion and so really knows what he is talking about.