I think weight loss truly is a "one step at a time" process. You can't tell if a weight is impossible to achieve until you are reasonably within the range of achieving it. When I first saw this chart I thought it was ridiculous and impossible. But then I made it to my goal weight and realized that it would be easy for me to keep going. I felt great and healthy. When you are truly at the lowest weight that you can achieve without feeling deathly and sapped of energy, you know. But you can't judge until you really start to edge towards that weight. When I was 95 lbs, I still felt incredible, so I thought that it'd be easy to lose another 5 lbs. I was wrong though. I realized that the other side of 95 lbs was too low for me. So now I'm working towards 95 lbs again in a healthy and controlled manner, and I know that that is the perfect weight for me. But until you really are at that perfect weight, above it you feel like you could still lose a few and below you feel like death, you can't measure your weight loss based on what you see on Tumblr or by BMI or by what you think your favorite model weighs.
That was a weird and random rant, but I hope it helps some of you that are unsure of your ultimate goal weight at least.