I wish instead of using some formula they just used the Odometer reading from current and last charge. At least then everyone no matter what their driving habits are would get a real idea just how much range they have unique to them.
Think of it as a goal of what you could achieve if you change your driving habits. When I am easy on it I get closer to that figure. It is a challenge. I have come close. I only was able to get more miles than was charged once , but I was driving about the speed of a golfcart for most of it.