[Phil G4SPZ]

I only have a 2.500 volt reference, but it seems to work well. For testing higher voltage ranges, I connect the reference in series with a variable DC supply, wind it up to get a suitable voltage indication (say 10 volts) on the meter under test, then switch out the 2.5 volt reference and look for 7.500 volts on the meter.

Calibrating the current ranges is a little less straightforward. I usually apply a known accurate voltage through a known accurate resistor direct into the movement, and adjust the mag shunt to create the correct reading. Once this is done, most of the ranges tend to fall into line, and any that don't can be swiftly identified and fixed.