This is a tweaked version of a calibration print which I used at a previous workplace. It takes a bit longer to print than the standard 20mm cubes, but allows for more accurate measurements.
The base cross is for measuring the X and Y axis - there is a small arrow to indicate the front of the model and also the Y axis. The notches in the base arms allow for a precise measurement. With the cube, it's possible that you aren't measuring exactly perpendicular with the sides. With this model, the notches mean you are always measuring the length accurately.
From notch to notch is 100mm. If your measurement is out, you will need to adjust your steps on that axis in your printer's firmware.
The Z axis is measured from base to top, diagonally across the square. That should also be 100mm.