what is the more important when changing saddles of different lengths the position of the back of the saddle or the nose of the saddle ie if you are replacing a saddle that seems to be close to the correct set back with one that is longer by 8mm should the new saddle's nose or back of the saddle be used for positioning it? I am using the stem bolt (quill stem) as a measuring point. seems like it would be the back of the saddle but not sure. thanks