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 degrees. This gives you a 2* window either way: you can set the power timing in the range of 32 to 36, and the idle-timing will follow along, plus minus 2 degrees.
The big deal is setting the rate of advance to not have it come in too early and cause detonation problems, yet fast enough to tap into as much power as your engine is capable of giving, at the all-in point, that you have selected. Since your engine is rarely if ever held at WOT at one particular rpm in the range of 3200 to 3600, I see no good reason to seek perfection in that range, this early in the tuning game. So just keep throwing springs at it until the all-in occurs in the range of 3200 to 3600, and go drive it.... with the V-can disconnected. Get the post 3800 power-timing figured, then the Part Throttle acceleration from stall to all-in, with various medium to heavy throttle settings, and finally WOT.
After you are sure the engine is detonation free under these conditions, then
you can start playing with the Vcan.
Ok, I see I misunderstood your post. Hang on; answers are inside the quote, Click to expand. answers in blue
Does that help?