You can easily guess how much initial advance your engine wants once you have it up and running (I'd do that AFTER cam break-in though).
The vacuum advance should be disconnected while you do this.
With the engine warm, advance timing as long as the manifold vacuum keeps rising (you need a vacuum gauge connected to the intake manifold, the carburetor's manifold vacuum port, or the brake booster). Keep idle speed in check around a fixed value (somewhere between 750 to 950 depending on your cam) using the idle speed screw on the carb.
The advance setting which gives the best vacuum (checked with a timing light) is the ideal initial timing you should be using. Write it down. Depending on your camshaft, compression, etc., it could be rather high.
Once you have done this, back down the timing to something like 10°, have someone bring the RPMs to 3500, rotate the distributor until you have 36° (SB) or 38° (BB) advance, let the engine come back down to idle, and check how much advance you have now.
The difference between the ideal idle setting you got with the vacuum gauge, and the current value (after setting max advance), is the amount of mechanical advance you must
REMOVE from the distributor.
To do this, you will have to limit the size of the slots on the distributor plate (Mopar) or use the proper bushing (MSD distributor, for example).
Do not reset the initial timing to the ideal value until you have limited the mechanical advance in the distributor.
After that, vacuum advance should be connected to manifold vacuum - or at least, that's what can be rad on the last page of
this document among other places.
The 36° and 38° values are universally known values, I guess engine specifications, elevation, and fuel quality might require a few degrees more (or less).
A part-throttle drive uphill or a WOT pass should be made confirm that there's no pinging. In case of pinging, back off 2° and retry.
I know some people use a different method for initial timing (advance as much as possible while still being able to start the engine without experiencing starter kickback). that's kind of race only though. A little bit like locking out the distributor and running 38° initial with a 20° retard circuit for starting.
For example, I have a SB with 24° initial and 12° mechanical advance. I'm rebuilding a BB and I'm guessing initial timing will be close to the same.
You will also have to set up the springs in the distributor to decide at which RPM you want the total timing.
