The stock z steps/mm is slightly off on the default z steps per mm from the factory (405 vs 400). The correct value given the type of threaded rod used should be 400. The math is easy:

  • 16 (times micro stepping); 200 (full steps/rotation); 8 (thread pitch)
    16 * 200 / 8 = 400 steps/mm (The shipping default is 405).
  • Set this new value through Repetier Host or Octoprint with the EEProm plugin. Without this change your parts will grow 0.0125mm for every mm in Z which equates to about a .5mm error for every 40mm in height.