For those interested, here is a link to a report commissioned by the Irrigation Association with the EWRI and ASCE to develop a standarized reference ET equation. This report includes all of the formulas needed to develop the data needed to compute the reference ET.
http://www.kimberly.uidaho.edu/water/asceewri/ascestzdetmain2005.pdf
The NWS has climate reports are available for free from thier FTP site, which may be an option if you live relatively close to one of thier observation sites.