How to calibrate the spirit level？

How to calibrate the spirit level？

The calibration instrument is composed of an optical indexing head and a special fixture. During calibration, the special fixture is fixed in the spindle conical hole of the indexing head, the indexing head is adjusted to make the flat plate roughly horizontal, and the spirit level is fixed on the flat plate, as shown in the figure, and then the calibration is carried out item by item.