Some additional remarks: The current implementation of the OpenSprinkler-Weather script doesn’t set the watering percentage to 0 if it starts raining. This is due to the bad weather forecasts in regards to rain prediction. At least for Netatmo PWS there is a workaround to stop watering if it starts raining.
A single IFTTT applet: Rain detected – Stop sprinkler operation is used to stop any sprinkler activity if it starts raining.
The corresponding API command is: /cp?pw=xxx&pid=0&rd=3 (Set a 3-hour rain delay)
In general the rain detection depends on how fast the rain gauge will be filled with rain and how fast the IFTTT applet is reacting. This can take some time, if e.g. there is no remarkable rain falling. In worst case scenarios there is always a chance of running the sprinkler in parallel to the rainfall. However, my experience shows that using weather forecasts for stopping the sprinkler are not conducive. Especially in Germany weather forecasts sometimes predict rain during the day, which never results in any rainfall at all.