How to determine the RPM for all in?

With an automatic, you can get away with chit. But get your T-port sync set first, then let your idle timing determine your idle-speed, without changing the speed screw.
If the car seems kinda jumpy with the timing all in so early, then slow it down some.
If the choke doesn't work right,with the vacuum advance plumbed to the spark-port try a little less idle-timing.(See note I, below)
Sometimes slowing the All-in down, to in the range of 3200 to 3600,will allow the engine to run the next lower grade of gas.Yes you will lose a few horsepower, below that, But your combo is way too heavy on low-rpm torque already,lol, and you'll never miss it,
and you only go through that zone once at WOT, on the way to somewhere else.
At Part Throttle, you won't notice it either, cuz if you need a few more hp you just drive a little deeper into the throttle.
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note1
Why less. Well if you have a lot of idle timing then the T-valves will be nearly closed at idle, (see note2). And so, on the fast-idle step, the sparkport will not be engaged, and the engine will have to warm up on the Idle-timing only, with a couple of centrifugal. So in your case 20 +say 4=24. But the cold engine might like 40 or more. I used to be able to get Vcans from about 9 to 22. So to solve the on-choke problem you could run say 14 Idle plus 4 Centrifugal, plus up to 22 in the can for a total of 40*,imagine that. This would allow you to slow the fast-idle down to the lowest rpm that will keep the sparkport alive.
Note2
sometimes so far closed that the engine will want to stall when the throttle is abruptly closed.
Note3
My 367 with a 223* cam, ran 14idle/28@2800/34@3400/+22VA. With aluminum heads it ran a 10.9Scr on 87E10. Since it spun 295s to over 60mph, I didn't miss a few foot-pounds below 3000rpm. And over 100,000 miles I saved a lot of coin, not buying top-grade fuel.
EDIT:
To run the 2-stage curve, I had to grind a buncha metal off my flyweights. My spring selection was very limited. I ended up with several sets of different weight,weights; as well as several different cams; some with modded slots.
To make all this, fun, I got rid of that annoying clip at the top of the driveshaft. Instead, I drilled and tapped it to receive a tiny machine screw. Then I rooted around my junk drawer and found just the right spacer to put under it to keep the cam from walking up the shaft. This made changing the curve a breeze.
I also set my power-timing a couple of degrees shy of what might be considered optimum.My little 367 makes gobs of torque, and with my TM (Torque Multiplication), I don't need every last bit, so again, 87E10 for the win. I think I ran close to or over 190psi (10.9Scr),.......with a [email protected] cam,..............still on 87E10.