An alternative option is to run a script that periodically sends command to OpenSprinkler to update its watering percentage parameter. This is probably an easier route as there is no need to change the weather script url. Instead you will use a script that calculates the watering percentage the way you want, and send it to OpenSprinkler through one of the HTTP GET command (check OpenSprinkler API document for the corresponding command and parameters).
I’m waiting until the weather adjustment is sorted out, the weather providers keep crippling the API’s to their data services.
If anyone can point to instructions on how to “run a script that periodically sends command to OpenSprinkler” I’d be grateful.