You all know the calibration cube. (20x20x20mm)
It's quite good and beautiful. You set your values, flow, etc.
Prints the cube. measures - perfect.

Now you print something larger. Suddenly the dimensions no longer fit exactly. The fits are inaccurate, or worst case, the sides of your box are not square.

The problem: The Calicube is simply too small to achieve a high level of measurement accuracy. The angular deviation from 2 sides is hardly detectable.
The length deviation and angular deviation can only be determined over a larger distance.

Here is my calibration part.
You can check your printer for perpendicularity of the X and Y axes to each other. In addition, the length deviation over a greater distance.

The squareness can be checked either with a protractor. Or you can measure the diagonals of the square. These must be exactly the same length.

You also have the option of checking the length accuracy of 50 or 80mm inside and outside dimensions.
The attached PDF shows the square as a technical drawing with the exact, relevant dimensions.

Calibrate your printer - now! :)