RE: ET from a solar sensor and irrigation. There are 2 ways to use it.
The ET output from a Davis console is summed over 24 hours from midnight.
Sum the 7 day daily max ETs and get a good estimate of the amount of water you should irrigate next week in inches.
I developed that and called it the Irrigation Index.
Then I found that the California DWR had developed the Watering Index. To use it you need the above summed daily ETs, as well as the highest average weekly ET for your location, and an irrigation timer with a Percent adjustment.
Set your controller to water the amount of the highest average weekly ET.
Then, the current controller percentage setting = (Last 7 Days ET/highest average weekly ET) x 100. Note that for a hot, dry, windy week, the setting can be > 100%.
Also, I modified that DWR index to include last 7 days rain for SLOweather.com. ((Last 7 Days ET-Last 7 days rain)/highest average weekly ET) x 100
Note that this modification can go negative. I wrote a PHP script to do those calcs for display on SLOweather.com. I think my Irrigation Index also includes the last 7 days rain as well.
Here's a grab of today's Indexes: