Ensure free camera lines up behind player

I have a free standing camera right that I'm trying to line up behind the player, so the camera is always looking at the player, but set back just a little. It almost works, but the more I move the more it is offset behind the player. The parent camera right should always line up directly behind the player.

Hierarchy is setup as follows:

-Free standing cam rig
----Main Camera (child of free standing rig)
-Player

 //rotate player based on mouse
 player.transform.Rotate(0f, rotationAngle,0f);
 

 //Set camera right behind player
 parentRig.position = targetPosition;
 
 //Rotate rig around player
 parentRig.RotateAround(player.transform.position, player.transform.up, rotationAngle);


 //rotate camera in rig to look at character. Ideally we'd be lined up directly behind player, but if we need to offset the rig for panning purposes, this is here. 
 transform.LookAt(lookAt);    

If I move forward/back/right/left without the mouse, all works ok but with mouse movement it just seems to start getting offset from behind the player - ie the camera rig no longer lines up behind the player when the player rotates and ends up looking at the player from an angle. The crosshairs on the screen then don't line up with where the player is looking since they are gui based and the player is no longer centered to where the camera looks.