You can accomplish what you want without touching any files or modifying things directly. Both the OpenSprinkler and the interval program for the OSPi support an API for changing settings. For the watering percentage it can be modified using the following command:
The above code will change the watering level to 250%. The range is 0-250. This is a direct multiplier on the minute duration for the stations.
With this information it is absolutely possible to augment my already present weather based rain delay to get more control. However, you need more robust weather sources than Yahoo. For weather underground an API key is needed which is why I haven’t switched to this service. I also don’t have an algorithm to base my program.