On some pictures in his books he sits really far behing the withers, but when you look at the other pictures, you see that most of the time he sits approximately at the spot where you 'glide' to on your own anyway. As Danee wrote already: it's hard work sitting on a spot too far forward or too far behind!
Hempfling's idea behind sitting so far to the back is that in dressage your horse is taught to make his frontquarters lighter and raise them while loading and lowering the hindquarters. So his conclusion is that is you sit further to the back, you make this easier for the horse. It sounds logical, but when you look at the anatomy of the horse you see that the point about 10 to 30 cms (depends on how big your horse is of course
) behind the withers is the point where the spine is the strongest (with long, dorsal extentions) and where the musculature (that you sit on) also is the strongest. If you're going further to the back, the structure of the back weakens and you also risk sitting in the area of the kidneys, where the back is quite vulnerable.
So the idea of Hempfling to sit further back is logical from a dressage point of view, but not so much from the biological point of view: the horse is built to overload his frontquarters all day long when grazing and standing and walking around on his own, so the frontquarters are actually designed to deal in a healthy way with a lot of weight resting on it.
So I would just sit on the place that feels the most comfortable for you (which is approximately at the spot where you would sit on when in the saddle too).