Hey Steve, I am planning to install Aprilaire 500M with a manual humidistat control (actually, I will not use its humidistat at all)
I will be wiring the relay with humidifier’s solenoid ( I will be using Dan’s [RELEASE] ST_Anything v2.9 - Arduino/ESP8266/ESP32 to ST via ThingShield, Ethernet, or WiFi based relay) and get the humidity data off my Honeywell T-stat.
You can use NEST Weather from NST Automations (https://github.com/tonesto7/nest-manager/blob/master/)