Steps, the motor is a 71 but I don't think that's much different from a 69 model so I'll work with your ideas too.
It doesnt matter if it is a1908 or 1990, its about having or not having the EGR and mid 70s pollution stuff hooked in.
basically Pollution stuff has a low cent degrees and very high VA degrees on ported vaccuum, and non polltion has over 20/ 22 degrees in the cent and around 7 to 10 degrees in the VA on manifold vaccuum
Ball park u are looking at around 8 to 12 degrees of intial...easy fire up low loading of the starter armiture.
Engine fires and the VA is all in giving a stable idle around the 16/ 18 degrees... over 20 for big cams... the VA should be all in at intial advance manifold vaccuum (no more than 1" above) with mixture screws adjusted to best idle temporially...generally VA starts about 5 to 7" and finishes around the 12 to 13 " ...
Total,(intial+cent) on modern fuels, engine designes, cams etc, will be in the 32 to 36 degree range around the 3000/ 3200 rpm range
The amount of degrees in the VA should not exceed 42 degrees (intial+cent+ VA) unless dialing in timing with knock sensor.
Filling / rounding off the counter weight tail wher contacts the anvail increases the degrees in the cent...about .5 to 1.5 mm...
Changing springs changes the rpms the cent (total) degrees.
Filling out the slots, or using stops mounted under the VA mounting screws, top or bottom end as required changes the VA deg range and start / finish pionts.
'adjustable' VAs change the tension and the amount the diapham moves at the same time, and are not always adjustable enough to have enough vacuum and number of degrees at the same time.
Dialing in a Dizzie is locking the counter weights, no VA ....
the best idle advance and using the intial and what is in the VA to achieve this 'tuning piont'
The best total (power curve) is established.. starting around 36/ 37 degrees the runing power runs and retardd till u feel the power drop off suddenly, then ad 2 or 3 degrees..
Then adjust the cent, VA , springs to work at those esblished tuning points.
To dial in for best results the carb idle, powe circuit and cruise circuits need to be in ball park 1st...
idle and cruise around the mid 15s AFR and power mid 11s to mid 12 afr.
Timing/dizzie is a hi precision instument...to have a given engine max cylinder pressure, explosion at a given point
(not in the range) somewhere between 12 and 18 deg ATDC...change compression, rpms, bore stroke ratio , mixture, fuel type or octane, and the timing requirements change.