Hi Steven, in regard to the Zimmerman method, I am going to expose the humidity weight, temp weight and precipition weight. This will allow users to use a different percentage modifier for each value.
These options will extend to other methods and be unique to each method. We plan on having this released for 2.1.5. It will work by detecting the selected method and showing the available options. The options will get stored as JSON and will be sent to the weather service when requesting the weather adjustment. This way things work as they do now with the benefit of adding per-user customization to the adjustments. Right now, we only have Zimmerman and I only plan on exposing three configuration options (temp, humidity and preciption). If you make your own method, just tell me which user configurable options you would like and we can go from there.
In regard to the Weather Underground data, you can get all data you would ever need to compute a scale. Just look thru the current variables that are parsed in the weather adjustment and if any additional ones are needed, we could grab them. Code available here: https://github.com/OpenSprinkler/Weather-Adjustment