If the old leader is lost, the length of the leader needs to be determined. To determine the length on a German movement, look at the CM number off the back plate of the movement. This measurement is the approximate length from the top of the movement to the bottom of the pendulum.
So if you measure the pendulum tip to tip in inches and then add an inch for the suspension spring, the multiply times 2.54 it will give you the CM length of all but the leader. The remaining length from this measurement and the CM length from the back plate, will be the approximate leader length.