Squaring S3 XXL

[WillAdams (William Adams (Carbide 3D))]

Perhaps this thread helps?

What is square for SO3

Another method to use to square the bed is use a drafting triangle and four 5mm or 1/4" shelf pins. Drill two holes inline on y axis and two holes inline on x axis. Insert shelf pins in holes. Check square between your pins and drafting triangle. Use a feeler gauge to determine which way the machine needs to be square. Adjust diagonal to correct for 50% of error. Drill new set of holes, repeat. Just like any new machine, it takes a little while to set it up correctly. Patience.

There’s some more guidance and more links (including that one and just added @cgallery’s excellent writeup) at: Shapeoko CNC Router, Rigid, Accurate, Reliable, and Affordable