Originally Posted by
Leisesturm
If you will study your own chart that you attached you will see that it is very easy to get 50mm of "setback" with a zero offset seatpost because the frame already has a significant amount of setback built in. Setback, as they define it is the distance between the saddle nose and the bottom bracket.
As you pointed out, the setback in in fact the horizontal distance from the center of the BB to the tip of the saddle. I actually had realized that before and forgotten about it by the time I came back looking at this issue. Problem solved.