The rain sensor itself should have a way to adjust the rain delay amount. Many rain sensors have an opening that you can adjust the size to control the evaporation. Adding the extra delay is certainly possible in software, but it will incur other changes, for example, someone will want to maybe cancel the extra delay so the UI/app has to be changed to also accommodate that. I think it would be better to use a rain sensor that can provide the appropriate amount of delay itself.