I believe that I am the one that first introduced the Irrigation Index and found the Watering Index 7 or 8 years ago.
Background...
I started by inventing my own Irrigation Index. It's based on the assumption that people irrigate on a 7 day cycle. Basically, it's the sum of the last 7 days' daily evapotranspirations (ETs) minus the sum pf the last 7 days' rain (both in the same units, I'm in the USA and used inches). If it's positive, that's about how much water you should put in the next 7 days. It's especially useful for someone manually irrigating, or using an old controller. Ours is 25 years old.
Then I learned that the California Department of Water Resources (DWR) had published their Watering Index. It needs the highest annual 7 day average ET sum for the locale in question, and also uses the 7 day ET sum. Divide the 7 day sum by the high average and express it as a percent. That's the percentage you should set a properly adjusted modern controller to for the next week.
I then modified the DWR's index by subtracting out the 7 day rain from the 7 day ET before dividing, giving what I think is a more useful percentage.
The main issue with the DWR index is finding the high 7 day ET for a given area. If you can't find it, you could estimate it (around here I'd start at at least an inch a week, ours is 1.45") and refine the number as time goes on.
I did include these calcs in another script I wrote to calculate some other fire weather indexes, but is based on a VWS text file that is uploaded to the server. I just looked at the script and it is, unfortunately, way not publishable.
Still, this is a teachable/learnable moment. If I figured out how to, in my hacky way, get it done in PHP, anyone else can. All of the math is here. I don't even use a database. The PHP script uses the VWS uploaded text file and saved data in a separate text file from yesterday. Today's calcs are written over the saved data file for tomorrow's run. A cron job calls the script once a day just after midnight.