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.
Did you know that the cylindrical mouldboard is the most aggressive type?
A helicoidal mouldboard produces large clods which remain together with the line of turned soil. A cylindrical one, by contrast, is much more aggressive, pulverising the line of soil as it turns it, mixing clods with the fine earth. The universal mouldboard has an intermediate effect, producing lumpy results under the surface and a surface layer of smaller lumps mixed with fine earth. Slat mouldboards are most suitable for wet and limestone areas.