To directly answer this question
I can say that I have personally gotten to the point where there is the slightest of gaps between the edge of the needle and the edge of 'E' line mark. (needle has past the lowest part of the E line and longer has any overlap with it)
I don't have the guts to go beyond that point. Once I see a gap there, I stop at the first gas station I come across.

 

What a concept using a highly volatile fuel much in vapor form present as a coolant.

 

I usually fill up before the warning light comes on, but wound up go further than that today. The light came on last night, so I made the drive over to Costco today for a fill up ($.20 cheaper a gallon adds up!) Supposed to have a 10.6 gallon tank, put in 10.2 gallons today. For me that is cutting it very close.