Seat set back is the horizontal dimension from the tip of the saddle to the vertical line intersecting the center of the BB. Not referencing the seat post but is partially determined by post set back. I mentioned it because of what Eric said in his post
#5 .
I prefer a .6mm wall center section on the bikes that will see a lot of load or a lot of leaning against stuff (like when locking the bike). For practical bikes a tad of frame weight is lost in the total riding weight more often then not and years later more likely to be still round and ridden. Andy