How to make a "Third person character's" arm point in the direction of the camera?

The divide by 2 node was there simply because I was following a tutorial and that’s what he did. To be honest I’m not sure if it’s helpful in my setup because I’m trying to rotate the arm not the head (as in the tutorial). If it’s off by 45 degrees, why would I add or subtract 90? Or is that just to get an exaggerated result for testing?