Originally Posted by
dsaul
Saddle setback is measured as a distance from the center of the bottom bracket with the saddle at a set height. The frame makes no difference to the setback of the saddle. It may change where the saddle rails fall on the seatpost, so you may need a seatpost with more or less setback to accomodate your needs.
Since all saddles differ in shape, length, etc.. as well as where the rails reside in relation to the sitting surface, where on the saddle is the measurement to? Where the sitbones should be?