Hi DigiH - I came across your response while searching for ways to calibrate the rain total on the WS90 via RTL_433.
I previously had an acurite station connected via the acurite rtl_433 add-on. Once that station died I purchased the WS90, 915 version. I am able to recieve the data via MQTT and the original acurite add-on successfully, but the rain data is incorrect. It seems that there are some options to calibrate the rain sensor via the gateways. Are you aware of a way to do this via rtl?