I have always used the number of 17 rated miles below zero, although I have only tested it to a few miles below zero, and always plan to arrive at zero greater. Before they changed the battery display vs miles left, I extrapolated to no battery from the zero miles point on a graph and it came out -17 rated miles at no battery left (I assume still an anti-brick reserve left).
I have found in my MS that 290 Wh/rated mile is a good conversion factor. If you use that and the "Zero Mile" protection number from the illustration, then 5.1 kWh/290 Wh/rated mile is 17.5 rated miles. All pretty consistent.
As others have said, always be careful and always plan to arrive at 0 or greater miles. If you do push it, remember that rated miles are not actual miles. For example, because of elevation, even in the best of weather, and the most careful hypermiling, the last 9 actual miles from town (and a 70-Amp J-Plug) to my house in Pagosa requires 15 rated miles from the battery.
I am very aware of all of this when I decide whether or not to drive past the J-Plug and continue on to home. Yesterday, I did a 255 rated mile charge in Silverthorne and made it to home with 23 rated miles left after driving 239 miles. I had 38 rated miles in the battery when I passed that last J-Plug, but having done the drive home so many times, I was confident in not grabbing any more miles. If I had been at 15 or below in the battery, I probably would have stopped in for a few Joules at the J. It is only because I know this route so well and know that there are several reserve miles that I am willing cut it this close. I have never had to flat bed my S...and I plan to never be in that situation.
Yesterday was an ideal hypermile day, moderate temps and cross or tail winds most of the time. That 318 miles is from Boulder to Pagosa and 295 Wh/mi is as good as I have ever done for this trip. The 273 Wh/mi is pretty amazing from Silverthorne to Pagosa even with 1,000 feet of net elevation loss, crossing three passes, Fremont, Poncha, and Wolf Creek. I even set the cruise control at 73 mph in the San Luis Valley once I knew that I had Pagosa made, and hit
significantly higher speeds for several passing events on the two-lane highways with short passing windows. The first half of the trip, until I knew things were going so well, I limited the max speed to 60 mph.