Sounds like you’re on the right track. It needs to be fairly easy.
Some of the data used in the calculations don’t change over time such as location (Lat. and elevation). So there needs to be a one time configuration plus the ET value from the weather source.
I understand Australia has a system like the one here in CA and also I think Idaho does but that is still just part of the world.
Ray once mentioned that many of his customers are outside the U.S.
For the U.S., the NOAA has freely accessible data available:
http://www.nws.noaa.gov/view/national.php
There may be similar resources for other parts of the world.
This is where thing really start to get interesting.
Dan