"The International Committee of Weights and
Measures (CIPM) Consultative Committee on Electromagnetic Quantities (CCEM) has produced a
guideline which states that no action is required
if your voltage related uncertainties are larger
than 0.25 ppm5. If your uncertainty for the volt
is less than 0.25 ppm and you do not have direct
access to a JVS, we recommend that you add the
0.107 ppm adjustment quantity to your uncertainty analysis."
Unfortunately I don't have any voltage standards with calibration uncertainties below 0.25ppm