Evapotranspiration (ET) calculations

I would really like an answer to this question about the method used to calculate ET. Many methods have been developed over the year to calculate ET and some have been much more accurate in the Midwest.
Maybe rachio considers this a trade secret and if so, it would be great to know that as well.
I am sure someone on the software team will need to answer my question, but not sure how to get it to them.
Thanks for the help.