Great post.
To answer your question about when does the low fuel warning light comes on...it depends on which engine you have.
The data sent to the IKE (instrument cluster) by the fuel level sensors...determines when the low fuel level light comes on. TEST 6 can be used to verify your particular BMW...when the low fuel level light/warning comes on...then open TEST 6 and look at the reading of the right fuel level sensor...it should be either 8 liters (I6 engine) or 10 liters (V8 engines). This is your "reserve" amount which is mentioned in the owners manual: