The measurements mentioned, e.g. 55mm setback, is from the middle of the bottom bracket to the tip of the saddle. The measurement markings on your seatpost are only for you to keep track if you move the saddle forwards or backwards, it's not relative to any other part of your bike.
The measurements mentioned, e.g. 55mm setback, is from the middle of the bottom bracket to the tip of the saddle. The measurement markings on your seatpost are only for you to keep track if you move the saddle forwards or backwards, it's not relative to any other part of your bike.