It sounds like this would require customizing the weather algorithm. At the moment we haven’t provided the ability to directly customizing weather algorithm through the app. However, you can change the weather script url to point to your own server, where you can host a custom weather algorithm. To change the weather script url, you can use http://x.x.x.x/su (where x.x.x.x is your OpenSprinkler’s ip address). To customize the weather algorithm, as a starting point, you can look at the existing weather script at https://github.com/OpenSprinkler/OpenSprinkler-Weather.
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).