Align AR perpendicular gravity?

Hey guys,

The title pretty much says it. I am currently using an Image Anchor to to launch a large scale AR overlay (100m x 50m), and what i'm finding is that slight variations in the Image Target's detected orientation make the AR skew drastically as you move farther away from the target. So, i realized that i really only need the Image Target for its position and its rotation about the Y axis. Rotation about the X and Z axis I would like to be fixed based on the detected orientation of gravity. Does that make sense? It seems like it shouldn't be difficult, but i've been unsuccessful in my attempts thus far. I have set the alignment to "Gravity" and have just been modifying the GenerateImageAnchor example.

As a side question, what role could/should SetWorldOrigin play in this situation?

Thanks everyone!