Hello djrobx,
The type of irrigation that you are referring to is called Evapotranspiration (ET). The system you are referring to that is run by CIMIS in California provides the needed information to run Evapotranspiration type cycles.
While the ET based systems are theoretically the most optimized form of irrigation, in practice they are difficult to implement and don't always provide optimization. Example: ET was designed for agriculture to provide growers with irrigation information for optimizing their crop yield. The formula takes weather elements and crop type into account. The method is based on assumptions that hold for agricultural irrigation environments but don't hold for most residential irrigation installations. That is: 1. the "crop" is constant; 2. The irrigation area is uniform. In residential irrigation, there are many "crops" - lawn, garden, trees, shrubs. In residential irrigation the irrigation area is not uniform: tree and building shadows, different soils.
The basic assumptions upon which ET is based for agriculture are not valid for residential - in most cases and so this form of irrigation isn't as optimal as some would have you believe. To make ET work you need additional information: inches of water applied per minute or hour; and a good weather station with a rain gauge. Also, the CIMIS system doesn't account for microclimates so you have to make adjustments to the formula.
In residential use ET is complicated, requires additional information that many installations don't have, and requires adjustments. Our monthly setback function is an ET estimation. It is a simple number system that most can apply. It is a method recommended for use by the water utilities in Southern California who publish a monthly setback number on their website. It is a practical form of optimization.
With that said we are looking into how to use the CIMIS data within California for full ET based cycles.
Jim