How do I convert a point in the world to coordinates on a world space canvas?

Hello Everyone,

I've got quite the stumper today (for me anyway). I need to be able to place an image on a canvas to mark where something in the world is. And since I am working in VR, the canvas has to be in world space mode.
I've got the code below which almost works, however it is very.. floaty.. it will be on the target if I am looking directly at it, however when you look away it drifts off the target in the opposite direction you look. A cool effect, but not what I need.
Here is the Code:

 screenPos = cam.WorldToScreenPoint(worldPos);
 canvasPos = new Vector3(screenPos.x - cam.pixelWidth/2, screenPos.y - cam.pixelHeight / 2, transform.localPosition.z);
 transform.localPosition = canvasPos;

The canvas is directly in the center of the screen, and the image (which the script is attached to) is anchored to the center of the canvas.
Any ideas on how I can fix this?
Image of drifting (aimed at the corner of the terrain): https://i.imgur.com/EGwFYaX.gifv
Unity 2017.3, OpenVR/SteamVR, VRTK, Windows 10 x64, WMR