I am trying to get the angle between a mesh (plane) and the main camera, because I need to fade out the plane when the camera sees it from a certain angle.
What would be the easiest way to get the angle between the mesh and camera? Thanks for this one, cause my math sucks. 
just learned this one myself: Vector3.AngleBetween
hereโs the variable from my script:
var fov = Vector3.AngleBetween (transform.TransformDirection (Vector3.forward), playerDir) * Mathf.Rad2Deg;
you have to include that last bit (multiply by Mathf.Rad2Deg) to use degrees.
the full script is in this post:
http://forum.otee.dk/viewtopic.php?t=1862
and the reference for it in the doc (bottom of page):