Get vertical FOV of Google Cardboard stereo camera

I'm building a google cardboard app using the Google VR SDK for Unity. I am stuck trying to access the vertical FOV of the stereo cameras using the following code:

 Camera cam = GameObject.FindWithTag("Camera_Left").GetComponent<Camera>();
 float fov = cam.fieldOfView;

This works in the editor, however when built to an android phone it returns the wrong value. Any suggestions on how to access the correct FOV would be greatly appreciated.

Thanks in advance!