This model is what I designed to help me calibrate the steps/mm and alignment of my printer. I found that the error on a standard 25mm cube was greater than I liked, and I'd often forget how it was aligned, so I wouldn't know if the corrrections applied to the X axis or the Y axis.

This model is 100mm in each direction, and has axis markings on each arm.

1. Slice and print the model (no particular settings, but I went with at least 0.6 infill, just to try and reduce how much it could flex)
2. measure each axis,
3. divide 100mm by the length of each axis, to get your correction factor, to be applied to the steps/mm for that axis.
4. Check the alignment of the axes against a set square. if they are not at 90°, you need to check the alignment of your rails, and adjust accordingly

My first test with this print had X=99.7mm, Y=100.02mm, Z=100.00mm. Pretty close, I'd aim for no more than 1/1000 error (i.e. 99.9-100.1mm). For the angular alignment of the axes, my XY was spot on, but XZ and YZ were both off by about 1mm over the 100mm span; should aim for less than 0.5mm over the 100mm span.