Check if rotation (only on Y axis) of some object toward another is finshed?

Hi there. I already know that simmilar questions have been asked before but it´s kinda different from what Im trying to achieve. In my case the two gameobjects in question (attacker and target) could have different Y positions (different heights) see the Picture below (Imagine that the Unit B is a flying Dragon in this case) :

Each time a unit attack, first it face the target and do damage then the target face the attacker after the damage has been done (simillar to a classic Turn based game)

In order to make Units facing each other, here is my rotation code :

 public void Step()
 {
  Vector3 targetPoint = this.observerObject.position - this.targetToLookAt.position;
  Quaternion newRotation = Quaternion.LookRotation(-targetPoint, Vector3.up);
  newRotation.x = 0; // to rotate only around Y axis 
  newRotation.z = 0; // to rotate only around Y axis 
  this.observerObject.rotation = Quaternion.Slerp(this.observerObject.rotation,newRotation, Time.deltaTime * this.rotationSpeed);
 }

And to check if the rotation is done, I tried first :

 if (Quaternion.Angle(this.observerObject.rotation, newRotation) <= 0.1f)
 {
   finished = true;
 }

And also :

 if((newRotation.eulerAngles - this.observerObject.rotation.eulerAngles).sqrMagnitude <= 0.1)
 {
   finished = true;
 }

Both approaches works fine but only when Unit A and B have the same Y position. Any help would be greatly appreciated.