You need to set the timing with the advance mechanism fully advanced; a locking washer alows you to hold this position. The locking washer is somewhat special, however. The OD must be small enough so as to not hide the mark on the contact breaker cam and the ID must be big enough to fit over the center post of the advance mechanism. So, you may have to grind down the OD of a large washer in order to meet these criteria.
Or, you can do what thousands have done: buy a Boyer.