I compared Wunderground with Openweathermap on that location just for interest and there is a bit of difference..
https://openweathermap.org/city/5341145
https://www.wunderground.com/forecast/us/ca/cupertino?cm_ven=localwx_10day
I checked a nearby Australian site Oberon, NSW
https://openweathermap.org/city/2154537
https://www.wunderground.com/forecast/au/oberon?cm_ven=localwx_10day
I checked my nearest local Canberra, AU with OWM, WU and our local BOM, was a little bit out, but not +10C
Keep in mind the Zimmerman model calculations on the Opensprinkler weather app is running on predicted temps.
I reckon I’d just adjust the temp baseline and tweak your watering times,
It’s not like you’re running a nuclear reactor 😉