Calculating angles to a target and turning towards it

I'm making a game where my object needs to rotate to face towards a target. At the moment I'm using trigonometry to determine whether to turn left or right, like so

 Vector3 TargetPosition;
 
 Update(){
    Vector3 relativePosition = transform.rotation * (transform.position - TargetPosition);
    float tanA = relativePosition.x / relativePosition.z;
    if (tanA > 0f) rotationVector.y = 1f;   //turn right
             else rotationVector.y = -1f;   //turn left 

However the Object always seems to rotate 45 degrees of the quadrant.