It’s a diamond, stacked on a circle, stacked on a square. I worked up a file at:
The big thing is, you need a measuring tool which has 10 times greater precision/accuracy than you are trying to adjust to when calibrating.
The screws on the HDM should be very accurate/precise for most purposes — usually tool deflection and issues of feeds and speeds are the culprits for things not cutting as expected.
Hopefully a test cut in a suitable material will yield a result you find acceptable.
That said, the folks who are working to the highest degree of accuracy on our machines seem to leave a defined roughing clearance when making an initial cut, measure things, then adjust the file and finish the cut w/ a finishing pass which brings things to their final dimension.