Framing World Space Canvas

Hi all, I'm trying to find the position / rotation of my camera so I can align it perfectly with a world space canvas.

From other posts: https://docs.unity3d.com/Manual/FrustumSizeAtDistance.html

http://forum.unity3d.com/threads/fit-object-to-screen.36398/

I've written the following code:

     Vector3 FrameCanvas()
     {
 
         // Calc the size of the canvas
         Vector3 bounds = RectTransformUtility.CalculateRelativeRectTransformBounds ( worldSpaceCanvas.transform).size;
 
 
         float distance = bounds.y * 0.5f / Mathf.Tan (Camera.main.fieldOfView * 0.5f * Mathf.Deg2Rad);
 
         Vector3 targetPosition = worldSpaceCanvas.transform.position + (distance * -m_speech.transform.forward);
 
         return targetPosition;
     }

..and it absolutely does not work :-) Can anyone point out where I'm going wrong?

Many Thanks