You might search on this forum for the topic, it has been discussed a lot over the years.
I'll try to summarize for you:
* First, you don't/can't set initial timing as an independent thing from total timing. The two are inexorably linked together. Changing one *will* change the other.
* "Both" have the same overall objective, which is to set the maximum advance that the engine will "see" under load, at wide open throttle. However, since it's practically impossible to create that condition (engine under load at wide open throttle) while it's sitting still in your driveway (while you're under the hood and fiddling with things), we take advantage of the first fact above- that the two settings are linked and changing one affects the other.
* Vacuum advance: It's purpose is to add additional timing under part throttle, not under full load conditions, because that tends to help with fuel economy. This however is a secondary concern and is not part of the discussion. Any time we're talking about setting total timing, optimum timing for max performance, etc. It means that vacuum advance is NOT part of the conversation and thus should be disconnected and plugged so that there are no leaks. Remember, at full load wide open throttle, the engine is producing almost zero vacuum anyway - so the vacuum system won't be doing anything under those conditions.
Ok now: Like John23 said, every engine is different. Even "identical" engines (same make, model, year, and build details) won't necessarily all have the same "best" settings for advance/timing that will produce the highest torque/power output. For any given engine, the only way to find the "best" setting for it is through testing. This usually means something like repeated runs at the track (with very accurate time clocks) or repeated dyno pulls -- with each pass/pull made under as near identical conditions as possible (same air temp, same humidity, same fuel, same beginning engine temp, same driving technique, same RPM range, etc.) The process involves choosing a starting point, then making very small changes to the setting until you find the one that produces the best results. To my knowledge, that's the only way to really nail it.
For Pontiac engines with open-chamber, cast iron heads, the "best" setting will frequently be GENERALLY in the vicinity of 35-36 degrees total, "all in" by 2500-2600 rpm. Closed chamber iron heads (like the 670's) generally will "like" more than that as will some aluminum heads. Some, with really efficient chambers (like the aftermarket heart-shaped chambers) may not want as much. When trying to find the optimum setting for your engine, you're actually chasing three different variables: 1) what's the total advance setting it likes and 2) how much advance is "in" your ignition system - i.e. is the range from minimum to maximum 30 degrees? 20? 15? 10? 3) How 'quickly' does the variable part of that "come in" - i.e. from idle when advance is at it's minimum, as rpm starts to climb does it reach maximum at 2000 rpm? 2500? 3000? etc.
On factory GM points distributors, there's a pin on the underneath side of the advance mechanism that rides in a slot that determines how much travel/advance it's capable of. To limit the amount of travel, you put different sized bushings on that pin to limit how far it can move in that slot, or if you need more --- you use a smaller bushing and/or grind the slot to make it "longer". You tune "how fast" the advance "comes in" by using heavier/lighter weights and springs on the mechanism.
People have been talking about 'dial back' timing lights. Such a light has an adjustable control on it that you move to 'change' the apparent position of the timing marks and also will have a meter on it for reading advance. The way you use them, is you move the control so that the timing marks line up at exactly TDC, then read the meter to see how much advance that setting actually is. Here's an example of one: https://www.amazon.com/INNOVA-3555-A.../dp/B000EVYH7W
For them to work properly (for any light to work properly, for that matter) you must be certain that the marks on the balancer actually DO accurately indicate when the engine is at TDC. Not all do. The outer ring on a factory balancer, especially an "old" one, can slip due to the elastomer drying out, aging, or just plain wearing out causing the marks to "lie" to you. Some aftermarket balancers can be off even when brand new, as can timing covers.
Now that we have all THAT out of the way, the process for setting "total advance" using either an accurate timing tape or a dial back light is actually pretty simple.
* Disconnect and plug the vacuum advance (to simulate the vacuum the engine produces at wide open throttle under load).
* Hook up your light.
* Have a friend slowly increase RPM until you can tell from the light that all the advance is 'in' (increasing rpm more won't cause the marks to 'move' any more).
* Either read the amount of advance from the timing tape, or from the dial-back light.
* If it's not where you want it, adjust it (same way you do at idle - loosen the clamp enough to allow turning the distributor, re-tighten, read again ---- repeat until you have it where you want it). You can change it while the engine is at rpm if you have 'big brass ones' but I tend to not like doing that myself
* Go make a pass/dyno pull and measure the results.
* Lather, rinse, repeat until you find the setting that produces the best numbers.
If you've ever watched 'Engine Power' on weekends or other programs where they build engines then dyno them, that's what they do. They'll start out at a timing setting that they know will be 'too low' and then slowly add timing until they find the setting that produces the best power numbers. For Pontiacs, generally I like to start out at around 30-31 degrees and then work my way up from there, a degree at a time.
Then, once you've found the optimum setting. Use your light again (vacuum advance disconnected and plugged) - this time with the engine sitting at idle. You're not going to adjust or change anything, you're already done with that. What you're doing is just READING what the idle setting is so that the next time you need to adjust timing (you've had the distributor out for some reason, for example) you'll know where to set it AT IDLE so that it's at the optimum setting without having to go through that whole process again at RPM.
Hope this helps,