It’s finally cooled down enough to venture into my roof space…
The R/C filter I’m using is a low-pass using 220Ω and 0.22μF, which according to a few calculators I used provides a cutoff frequency of 3.3kHz (anything greater than 3.3kHz should be attenuated below 0.7 x Vin which is under the “HIGH” threshold for an ESP8266)
I just happened to have those kicking about and didn’t get too precious with it, I could probably have used a larger resistor (2.2kΩ would cut off >328Hz) and still been perfectly fine.
I’ve got a 0.5 second de-bounce in software for the rain gauge as well to give me a 1440mm/hr rain rate maximum. (The world record rain rate ever recorded is actually 305mm in 42 minutes = 215mm/hr or 8.4in/hr)