  The measurement of the angle between the interferometer front mirror and the
diffracting planes is a critical aspect of the Si lattice-parameter measurement
by combined x-ray and optical interferometry. In addition to being measured
off-line by x-ray diffraction, it was checked on-line by transversely moving
the analyser crystal and observing the phase shift of the interference fringe.
We describe the measurement procedure and give the miscut angle of the
$^{28}$Si crystal whose lattice parameter was an essential input-datum for,
yesterday, the determination of the Avogadro constant and, today, the kilogram
realisation by counting atoms. These data are a kindness to others that might
wish to repeat the measurement of the lattice-parameter of this unique crystal.
