reading a high negative voltage with attinys running on batteries
I think if you connect the resistor bridge to Vcc then you will get more resolution and far fewer parts.







simulate this circuit &ndash; Schematic created using CircuitLab

You would periodically measure Vcc using the internal voltage reference...

https://wp.josh.com/2014/11/06/battery-fuel-guage-with-zero-parts-and-zero-pins-on-avr/

...and then measure the voltage on the analog in pin against Vcc. You can then calculate the negative voltage on C1 based on how many volts lower than Vcc the analog pin is at. 

(Ahh, no markdown tables in SE!? So ugly text...)

For example, if the analog in reads at 100% of Vcc range, then you know that C1 is at 0V.

If analog in reads at 50% of Vcc range and Vcc is 5V, then you know that analog in V is 2.5V. 5V-2.5V = 2.5V so the bottom leg of the divider is 2.5V so the Cap is at 2.5V*100 = -250V.

If analog in reads at 10% of Vcc range and Vcc is 5V, then analog in pin is at 0.5V so bottom leg of divider is at 5V-0.5V=4.5V so cap is at -450V. 

Here is a simulation showing the voltages with Vcc=4.5V and C1=-300V, which results in analog in=1.455V...



Here I show the resistors to be a 1:100 divider for simple math, but you could pick values that exactly match the range of the capacitor voltage range for better resolution. 

Note that as long as R2 is large, you can also count on the built-in protection diode on the analog in pin to shunt to ground if the voltage on the capacitor ever drops out of range....