Calculating the angle based on coordinates of Touchinput?

I want to get a plane with an arrow texture to rotate and point to where the player is touching relative to its axis. How would I go about calculating the angle based on what coordinates are given from the touch in Unity. Obviously not a match person here. :P

Do you really need the angle or is it good enough if the Arrow points in the right direction? If the arrow points in the forward direction of your plane you could just use Camera.ScreenToWorldPoint to find the world point you are pointing at and then do MyPlane.transform.LookAt(worldPoint);