If one is bending metal around a mandrel then some change has to be happening to the material to allow it to conform to the circular shape. The outer diameter of the ferrule is 18mm and it follows from this that the outer circumference will be 56.556mm. This is a difference between inner and outer of over 6mm. However the additional length needed, when measured, to close the gap is actually 3.1mm giving a true length of 53.372mm, which is closer to Pi X 17mm. Is this then the correct sum to establish the length of brass sheet needed?