The 0.0 point is always the same spot as long as your crumb tray doesn’t movie. The maximum area will change by a few millimeters depending on your machine, but generally 10.95” x 19.45” - as is stated in the tech specs, and literally dozens of places on the forum.
If you make your art board 20"x12" and place your art in it - it will show up in the GFUI in exactly the same spot.
It sounds like you’re trying to make a jig without actually making a jig - which is fine. I know, for example, that on my machine the right bottom corner is 1/4" in and 1/8" up from the edges - so that’s where I put the edge of my board, but that placement will change depending on the speeds I use - and my crumb tray doesn’t move. If yours does, there’s a boot design on here to lock it down.
If I’m unclear on what you’re trying to figure out, and that doesn’t address it, can you describe it a different way?