This should be a relatively simple fix.
1) Log the date/time that Rain sensor notification first went active.
2) Check the weather station report for that date/time.
3) Log the inches of rain fall reported. This is the 0-Time value that must be logged in a variable.
4) Just before midnight on that same date, check the weather station inches of rain and calculate the 0-T value versus the midnight value. Save in a variable for day 0 totals. Note: Depending upon the forecast, more frequent checks may be needed.
5) Check the inches of rain each day just before midnight and Log each value in separate variables. Continue this until the Rain sensor reports "Deactivated."
5) When the Rain sensor dries out and reports it's deactivation, Log the final date and time.
6) Check the weather station for the inches reported at that time and Log it in the final variable.
7) The sum total of all of the variable values = Water savings for that period of Rain Sensor activation.
8) Adjust the watering schedule to Irrigate as needed to make up for any overages or shortfalls.