2 ways you can approach this, one is put the arm more to the side, the other is use more foreshortening without changing the arm position. Both used the same fundamentals of anatomy, when you raise your arm, your shoulders, chest and back muscles are also moving together, if you are unclear to how it works, you can use more references and 3D models to help with the posing and angle. Study simplified muscle studies to get an idea of the general grouping and attachments of the body muscles.

