In the picture below, we can see a very good position of the shoulder just when extending the arm.
It is important that the shoulder is lower than the ear and more towards the jaw.
This will help you get greatly to gain more power in your stroke and also improve horizontality and hydrodynamics.
Be careful not to roll to much your shoulders when you do this.
Try to keep your extended arm on a rail which would lay parallel to your body and not under your body.
Swimmer: Andi Bocherer (triathlete)