The Two Inch Rule (Duke Williams)
The vacuum can should provide full vacuum advance at no less than 2" less than typical idle vacuum with the proper total idle timing, which is in the range of mid-20s to low 30s. High overlap cams should have total idle timing in the upper part of this range and lower part for low overlap cams.
Total idle timing is the sum of initial, plus full vacuum advance, plus maybe a couple of degrees of centrifugal if the curve starts at lower than idle speed, which is the case on some SHP engines. Check the specs!!!
If the engine has no vacuum can or it is dead, tie up the centrifugal with a rubber band and increase the timing to the above range. As you do this engine revs will increase, so use the idle speed screw to reduce revs to your target idle speed.
Now measure manifold vacuum and use "the rule" to select a vacuum can.