If you are referring to the monthly adjustment plugin, that was mostly a demo, but hopefully a useful one, of the new plugin architecture for the Python Interval Program. I have some (untested) Python code for ET calculations you might be able to use as a starting point.
You can download it from:
http://1drv.ms/1f45m2F
You might be able to start out by simply subtracting the measured rainfall from the amount scheduled to be applied and build from there.
Some other folks here have been working on weather based stuff but i don’t know she status of their work at this point.
I am interested to see what you come up with.
Dan