What you want to do is to set TOTAL ignition timing to the "happy place" for your engine and then just let the initial value land whereever it lands. With iron open chamber heads, "most" Pontiacs tend to be happy somewhere around 35-36 degrees total. Aluminum heads, and also closed chamber 670's, tend to like more than that. If you want to really dial it in, then the process is to start lower - say around 33 degrees - then sneak up on it a little at a time, testing with clocks, until you find the setting that results in the lowest E.T.
To check total advance you'll need either a light that has that capability, or a timing tape on your balancer that has degree readings up to at least 40 degrees. Disconnect the vacuum advance, plug the line, and take the rpm up to where you can tell all the advance is "in" (the marks stop moving under the light) - usually somewhere around 3000 rpm is enough. Once you figure out where your engine likes to be, then take your light and check where it is at idle (like you'd "normally" check timing). Don't change it, just look at it - that way you'll know where to set it in the future without having to work on it at RPM.