Hi,
We all (should) know that a standard digital multimeter (DMM) loads a
GM power supply too much to get an accurate reading. A cheap and
simple way to get a high impedance divider is to use one of the high
value resistors from a ionisation type meter.
The CDV 715 typically has 2.2x10^8, 2.2x10^9, 2.2x10^10 and 2.2x10^11
resistors. the 220M ohm (2.2x10^8) can be used in series with a 22K
ohm and the voltage across the 22K measured with a DMM on the 200mV
range. A dedicated meter can be made up with an LCD panel meter. The
ratio is not exact but the error is small, less than 1V in 2000 for
the module (high input impedance) and less than 5V (0.25%) with a DMM
(10M input impedance) the 2.2G ohm (2.2x10^9) resistor can be used in
series with a 220K. In this case the 10M impedance of the DMM causes
a bigger error, about 2% so a 224700 ohm (220K + 4K7) could be used
giving a 0.12% error. If an accurate comparasion can be made for
adjustment, use a preset as part of the 22K or 224.7K across the DMM
input and set accodingly. If you use a digital panel meter module
just use a 22K or 220K and use the adjutment on the meter module to
set it to the correct reading.
Don't forget to handle the high value resistors with gloves to stop
oil from your fingers causing leakage!

Robert G8RPI.