On my old Focus MK2 with a 2011 MK2.5 instrument cluster the petrol warning light was most times illuminated arround 100 KM left.
My current Focus MK3 does also have this adaptive system. On my car the petrol warning light is illuminated at about 90 KM left. If I drive further until the trip computer shows 0 KM left and the fuel gauge shows completely empty there only goes 45 L in the fuel tank. This basically means that there is still 10 L left.
Based on the comments of other Focus MK3 Petrol users this seems to be normal for the Focus MK3.