You can enable Rain skip on Fixed schedule, but not on Flexible schedule. Flexible schedule already takes into account actual and forecasted precipitation.
Based on Rain Skip FAQ, https://support.rachio.com/hc/en-us/articles/115010540788, Rachio will take into consideration the precipitation of last 24 hours, as well as the next 24 hours, and if it exceeds the threshold, it will skip it. If you put down about 1" of water, is this once per week? You might want to increase the threshold so that it doesn’t skip watering in that case.
There is also Saturation Skip for Fixed schedule. Here is the link for its FAQ: https://support.rachio.com/hc/en-us/articles/360000096188-Saturation-Skip-FAQ. This takes into consideration precipitation and irrigation amounts, and losses through evapotranspiration, to determine if an irrigation should be activated.
Are you considering the Flexible Daily schedule in the future?