This can just go around in circles until you fall deep into the rabbit hole. When you go beyond 4 1/2 digits of resolution each extra one is 10 times the work to do it right. Until you have a traceable stable known reference to work from you will never truly know what you are reading in particular at 6 digits and greater.
So at some point failing that YOU need to create YOUR Volt preferably as close as you can to the 'true' value.
For a basic home lab this can be as simple as pick your best meter and what it says rules the others. Worst option so buy/make a Vref.
Buy one of the AD584 based references making sure it has the 'known' voltages recorded on it (most seem not to have this currently). Works well offers a range of outputs and is stable against 10M and GOhm meters. Suffers some initial drift from stated figures over time and drifts in the order of 1PPM/C. Should handle a divider to 1V and maintain ok stability and accuracy of its voltage with a small load.
Standard Cells of various sorts. See
Rocking Horse Poo Requires a GOhm meter or preferably a Null Meter and several cells. Not practical but a stable over time now curio and a sanity check device for me - if you can buy one they are Cool. NIST is back from being shutdown
https://www.nist.gov/sites/default/files/documents/calibrations/mn84.pdfLM399 and LTX1000 based devices are POINTLESS unless you have a 'known' accurate standard or meter to set them with their initial accuracy varies widely. They will only be as 'accurate' as the device YOU measure them with but generally they are more stable than other References due to Temp but can take weeks to years to settle. They are a can of worms in the Rabbit Hole with you in free fall but always just out of reach
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Roll your own using the best initial accuracy Reference you can buy sensibly back to the Max series (Best Initial Accuracy) again unless someone else knows of a $10USD one that is better? I have Two in aluminum boxes running from LiPo batteries tweaked to give me an accurate 10V I am happy with for outside use. I do also own a Fluke 515A portable Calibrator which is great to 4.5 digits meters but not so good for 6.5 as it drifts more than the Max based References.
Use your best 'known' Reference to 'Null' out a separate supply with more grunt if you need it to avoid loading issues and test it's performance as you load it to see if it remains 'stable' as you load it. Feed that into a Hamon or whatever divider or op amp you like and test away being happy in the knowledge it is YOUR Volt
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