headSize calibration marker

1. Print this page at 100% / "Actual size" (turn off "Fit to page"; only the marker and ruler are printed).
2. Measure the 100 mm line with a real ruler. If it isn't 100 mm, the printer scaled the page: print again at 100% / "Actual size".
3. Glue the marker to stiff card and cut along the dashed line. It must stay flat.
4. In the demo, click Calibrate with printed marker, take off glasses, hold the marker flat against your forehead, facing the camera, and keep still.