How’s the default tilt angle calculated?

The default tilt angle is calculated based on the latitude of the site as follows: 
- If the latitude is <5º, the recommended tilt is 5º 
- If the latitude is between 5º-10º, the recommended tilt is equal to the latitude 
- If the latitude is between 10º-20º, the recommended tilt is equal to the latitude*0.9 
- If the latitude is >20º, the recommended tilt is equal to the latitude*0.85