Squareness Tool

I ran into issues printing a large flat panel on my Ender 3. The issue stemmed from the bottom rail being out of alignment. When I search for a Calibration / Validation tool none seemed to be stiff enough to use against a wood working square.

I added a small hole on one corner to indicate a reference corner. I set the corner on the lower left side of my print. This allows me to see where the error is.