DanPro (https://www.youtube.com/channel/UC4lLVkUGPtM5pnS3ZskvECg) answered my question with this post (https://blenderartists.org/forum/showthread.php?412404-Corrective-shape-key-driver-for-the-shoulder) and this tutorial (https://www.youtube.com/watch?v=wUs04SGUnVI).
Quote from DanPro in the BlenderArtists link:
The shoulder does not rotate on the Y axis for the standard Rigify Rig. It only rotates in X and Z and that is on the bones Local axes. I can't tell from the screenshot how you have the bone aligned so I cannot tell you which axis to use. (Turn on bone axes display.) You can find this out by using the hotkey R, then X,X, or R then Z,Z (if you are using Normal or Global Transformation Orientation). Change your variable to this other axis, once you determine which it is.
Also, change the bone to ORG-upper_arm.L instead of the FK bones. This way, whether you are using IK or FK controls to pose, the driver and corrective shape will still work. The ORG bones for legs and arms is the intermediate that switches between IK and FK, so this is the bone that should be used to drive corrective shapes. (In most cases. Exceptions always apply.)
One more tip, a human cannot raise their arm like in the screenshot unless they involve the shoulder. The shoulder needs to be raised whenever the arm reaches the T-pose position and above.
His rigging tutorials are the best I've found on YouTube.