The plate should have no effect on the Vacuum advance. If it does I would mod it or throw it away.
Most of our hotrods will run very nice with 20 to 18* in the mechanical advance. That is to say:
at 34* power-timing. The idle-timing will be 18 to 20 degrees less or 14 to 16. This gives you a 2* window either way: you set the power timing in the range of 32 to 36, and the idle-timing will follow along, plus minus 2 degrees.