HunterLab follows the recommendations set forth by ASTM E1164.
Referring to section 10.2.1 which states that the Full Scale Standardization shall be done using a White Reference Standard calibrated relative to the perfect Reflecting diffuser. This implies that the White Instrument Standard has unique calibrated values which would be substantiated by a Calibration and Traceability statement.
Referring to section 10.2.2.1 for 0:45 and 45:0 Zero Scale Standardization shall be done with "a highly polished black glass standard with an assigned reflectance factor of zero." This implies that for any highly polished black glass the reflectance factor is zero at all measured wavelengths. There is no calibration required to assign the reflectance factor at each wavelength. The reason why no calibration is required is that except for a few very expensive instruments maintained by the NMI's of the world the true reflectance of polished black glass is many magnitudes below a commercial instruments ability to measure it.