David, for the 3458A: if you would know the deviation in advance, you could simply key in the 'correctly expected' values and then adjust the 10V and 10k reference values.. Yes, the 3458A CAN be adjusted electronically! Afterwards, it would be a bit more difficult, because you can't easily change both these calibration constants.. you would need a detour by stable external 10V and 10k resistors, for which you would calculate the determined differences from nominal, and then re-calibrate the constants. Any other 8.5 DMM than the 3458A has the complication needing a complete calibrator for all modes and ranges to simply adjust the instrument (i.e 100mV, 1V, 10V, 100V, 1kV, and so on), whereas the 3458A requires 10v and 10k only. Verification (which is implied by calibration) is a 2nd aspect. It is very difficult to perform that, especially on a 3458A, and also requires full coverage of all modes and ranges. For volt-nuts use, I can recommend the 3458A only. Resign on calibration, but simply get known-good 10V/10k from some friendly European volt-nuts, and you're within a few ppm uncertainty, but w/o  certificate. Frank