Larger and more complex parts require greater accuracy in 3d printer calibration, especially with regard to laying down the first layer. This calibration part will help you ensure your print head is always the right height above your built plate, regardless of where it is in the XY plane.A level build plate seems to be the result of a skill you develop the more you use your 3d printer. Using a calibration print allows you to achieve greater accuracy than with a spirit level or the human eye. Remember to print this part at the lower end of your polymer's temperature range. Very runny PLA is more forgiving of miss-calibrated build plates.More discussion over http://www.woodsmokeworkshop.com/wordpress/?p=209