As the cut width becomes narrower it is also a good idea to reduce the working depth correspondingly, as otherwise the quality of your work will suffer. For a 14" coulter the working width is 35 cm and the recommended depth is 25 cm. For a 16" coulter the working width is 40 cm and the recommended depth is 28 cm. For a 18" coulter the working width is 45 cm and the recommended depth is 32 cm.