If you are trying to use manifold vacuum to apply the advance it's a MUST to use an "adjustable" unit and you are going to have to set the "all-in" timing it applies BELOW the lowest vacuum reading you're getting at idle or it's going to drop-out some timing when a load is placed on the engine (trans placed in gear/vacuum falls off some).
Despite how popular that method of tuning is, it's more of a "crutch" when you simply don't have enough compression for the cam right to start with. As much as I've tried over the years I've never been overly happy with running the timing clear off the scale at idle speed to make an engine happy. Can it be done, yes, with enough persistence and making more fuel available to the idle mixture screws you can get it to work OK with those methods.
Also keep in mind that the carburetor is equally as important in getting a good end result when things are less than ideal for CID/Cam/Compression. Engines making low vacuum at idle will need more idle bypass air, and more fuel available to the idle mixture screws......
