TBH, i don't care about the absolute accuracy of the voltage source, as i will be using my 6.5 digiit bench meter as the calibration reference. As such, i just want a voltage source, that is short term stable and adjustable to a resolution of around 0.5mV output.
It needs to have a reasonably low output impedance (say <1kohm (ish) so it can drive the ADC input circuitry on various different kinds of test pcbs etc
If i used two decent quality pots say one 10Kohm single turn, and a 500ohm 10 turn one, that would look to work. In effect, the actual pot setting to achieve the outpout voltage is un-important, but just needs to be noise free (can easily have a low pass filter on it as i have no response time limitations for the circuit etc) and stable over say 5min (after a warmup period) etc