Voltage divider puzzle

[JonW]

From memory, I think FR4 is around  20kV/mm.  You could pot the whole thing or even use transformer oil to remove moisture.

Some notes on high voltage design HERE

I would also put protection on the low voltage side, a Gas Discharge diode (GDT), the lowest breakdown is still around 100V, and these have very low capacitance so with your large value resistors it will keep the response time low.  Many TVS are in the tens of NF range and they leak!  If you are using a buffer, be mindful of the input leakage currents and capacitance as this will add error and slow the response time

[charliecoutas]

Many thanks JonW, I will study what you say.