One of the things you can do extremely accurately is resistor ratio's.
Conrad, one of the forum regulars has some great metrology ideas on his site, and one of the really neat ideas is the Hamon divider.
http://conradhoffman.com/electronics01.htmThe thing is with a bunch of resistors and anything that can precisely measure 0V (like you 6 digit meter or an opamp circuit adjusted for 0V with a shorted input), you can make an extremely precise divide by 10 resistive divider. It is easy to adjust to better then 1ppm, but whether the resistors stay matching to 1ppm for long, that is another matter. With this divider, you can start to scale voltages (ie you can compare the 0.1V range to the 1V range, and then the 10V range to the 1v range, etc. The divider when calibrated ends up with a very precisely calibrated 10:1 ratio of resistors - and in the process you also get an equally accurate 2:1 divider. You can use the divider in a bridge to do relative calibrations for resistors from milliohms to gigohms if you are precise enough.
The trick with the Hamon divider is to start with at least 10 identical resistors - starting with a pack of 100 is even better as you can select the best matches. The value does not matter much, but around 10K would be good.
In each of the upper resistors of the Hamon divider use 3 resistors per leg, and one resistor down the bottom leg. Choose the resistors so that each of the upper legs is 3 times the resistance of the lower leg to within 0.01% if possible. One leg will have the adjustment pot so it has to have slightly less resistance. Since all the resistors are the same, they will have the same power dissipation and so hopefully when they drift, they will all drift equally and the ratio will stay the same.
If you see that the meter ranges are relatively precise, then it will show if anyone has been messing with the range calibration or not.
If you are using a 10MOhm input resistance meter to measure the divider output, you need to allow for this resistance. A 10MOhm meter will reduce a 10x divider that uses 10K resistors by 0.1%. I do not know about your 6 digit meter, but many high end meters have input impedances well over a gigohm for voltage ranges up to at least 20V and if so, there will be little error measuring the divider output.