A simple voltmeter is a (large) resistor in series with a (low) current meter, as in your proposed circuit.
However, your ammeter requires a relatively high current for a high voltage sensor.
25 megohms and 1 mA will give a full-scale voltage of 25 kV, but the power dissipated in the resistor will be 25 watts, exceeding the 15 watt rating of the resistor. At your expected maximum of 20 kV, you will still dissipate 16 watts. You would be better with, say, 100 microamps for the meter and 250 megohms.