Where my camera is directed in degree. How to use arctan2?

Hi guys

I cant figure out how to use arctangent2 to calculate camera rotation on Z axis in degree.
I want material which will show me trougth DebusScalarValue node camera rotation relative to world coordinate system in degree.

Im pretty new to vector math and all that stuff, so make a lot of mistakes and stupid question…

Here (and in all similar diagrams) we always have coordinate system, regarding which we move or rotate something and calculate the results. But how to recreate that coordinate system representation in material for making calculation?