Usually, when we talk about total advance, it's for WOT high-rpm situations, somewhat like Nick uses on his dyno pulls. We don't think about what it takes to get there, how emission controls impacted these things, much less vac advance totals.
From the 1950s forward, the racers' orientation was to get "max advance as quick as it can happen", so they could have more power quicker in low gear ff-line. In some extreme cases, even locking the distributor for no-advance, all initial setting. Doing that, they got "consistency", which was important to them in how their engines responded.
In the realm of street driving, many more variables. Still "more and quicker" is better than most OEMs did back then. As they all aimed for that "max advance" optimized number. AND doing it for then-existing octane fuels being used. When I found the SAE paper on the Chrysler 413, for 1958, it was optimized for then-ethyl, which was 97 Research Octane (forget about current Pump Octane, which did not exist back then). IIRC, it stated that 34 degrees BTBC is what it was tuned for on that fuel?
More initial advance always means better off-idle response, but not enough that the total amount does not exceed the 34 degrees BTDC total. Looking at the '66 383 2bbl, it starts at 12.5 BTDC initial. The low-speed advance happens nicely by about 1500rpm, than tapers toward max advance at about 4300rpm, for a total of about 34-36 degrees BTDC. Add the lower rpm advance and the initial and it gets to be about 43.5 degrees BTDC at cruise speeds.
It was discovered that retarding the initial amount of timing helped with CO and HC emissions, so rather than 10 or 12.5 degrees BTDC, the base timing became morel like 5 degrees BTDC, then using the higher rpm advance curve to make up the difference, usually still hitting 34 degrees BTDC (or so) at 4500+ rpm. So, only lower speed performance was affected, while still having full power at WOT rpms.
Less dense mixtures (whether from leanness, part-throttle, elevated altitudes, or combinations thereof) are harder to fire off than a richer (more dense) mixture at WOT, so it takes more "spark lead" to make that happen by the time the piston gets to or near TDC. Which is where the vac advance comes in, to incrementally and variably increase the timing amount as needed. And THAT is where the seemingly wildly-high advance figures come in. Every engine and combustion chamber design has their own "sweet spot" as to how they respond to these things, too.
For example, on the '66 383 2bbl in our Newport Town Sedan, its timing was 12.5 degrees initial. From when we bought it, it liked ethyl/premium fuel better than the regular it was supposed to take, so we used premium as a matter of course. It ran well and had good power and highway fuel economy. I tweaked it a bit up to 15 degrees BTDC, with no problems of any kind. If it was at 12.5, the slisght move to 15 would increase the idle speed. But on my '70 Monaco 383 4bbl (with 906 heard form the factory), when I might increase the initial from 5 degrees BTDC to something a little higher, NO increase in idle speed. I figured that was due to the more-open chamber of the 906s compared to the more-wedge chamber of the earlier models. Never did confirm that, though, just a suspicion.
Typically, vac advance is configured for ported vacuum, which means it is low at idle (lower than the point where the vac advance mechanism is activated) and then approached manifold vac levels once the carb is past the idle system in rpm and air flow. BUT, my investigations concluded that the CARB has to be set-up to use manifold vac for the vac advance rather than ported vacuum. Has to do with where the transition ports in the carb throttle body are in relation to the throttle plates. And all aftermarket carbs are set-up to use ported vacuum for the vac advance, as the vast majority of OEM carbs did or do. These are my experiences in investigating such.
The observed problem with "car advice" is that it can be variable and you have to determine whom is credible and who is not. This can be a big issue, too! And what might have worked for them, from which they extrapolate to mean "always works" might fail with somebody else with a slightly different engine combination. Which is where you have to get a little bit brave and start investigating for yourself, to see what might work and what might not. Tweaks rather than big changes, for example. Against a backdrop of "factory specs for your vehicle".
As to vac and total advance settings, the OEMs have to account for all of those variables in how they set-up the carbs and distributors, plus emissions compliance. So, for some, those factory settings are "too conservative" as they generally work fine for others. This is where tweaking can come in to see if "a little bit better" might be possible.
Driving in hilly areas is different from driving on flat Interstates, too. An engine totally optimized for Interstates might clatter in the hilly areas, for example. So this is where vac advance tuning can come in, with an adjustable vac advance unit. In reality, at this time, I suspect that most of the vac advance units are adjustable, just that we don't know it. As the total advance number might be stamped on the mounting flange, the amount of vac to start and max-out the advance can very from engine to engine, hence the hidden adjustment inside the can. Insert an Allen wrench into the nipple on the vac advance. With a little turning, it should drop into and index with and Allen head socket on the vac advance diaphram center. It should turn to change the spring tension, affecting when things start to happen, as to vac level. Otherwise, as the OEMs used to be, a bradded center of the disphram holder and no adjustment.
I hope this might have explained some things. In any event, use the FSM specs as a starting point/default mode . . . tweaking from there as desired. A digital timing light can be a BIG plus to have, so you can watch these things happen in real time. I bought a dial-back Craftsman timing light years ago, inductive hookup for ease of use, too. It was much better to check things on the car rather than having to use a distributor machine for similar things! It was more expensive than a normal timing light, but well-worth the additional cost for the benefits it provided. And now, we've got digitals!
Take care,
Willis