Well, lets get a starting point. Let's say you have a very mildly built small block and you have the initial timing set at 5* BTC and it's idling at 700 rpm's. With the vacuum advance disconnected, bump the timing up to 7* BTC. DID THE MOTOR REV UP A LITTLE? If it gained rpm's, then back the idle rpm's back down to the desired 700 rpms. Advance the timing again to 9*. Same procedure. If it gets to the point where it starts hard (hard to turn over full warmed up), back it off to where it did not start hard. With a very mild small block, you will end up somewhere between 14*-22* initial timing. Some times the grade of gas will be a factor. Example: You may have to run 5* less if you have 9 to 1 compression versus 8 to 1 compression because of the gas to keep it from "pinging". No Pinging!!! lol.... at low rpm in high gear, slightly give it gas going up an incline, it should not ping. Neither should it "ping" under WOT. Pinging can be controlled by full advancement, vacuum advance, and how fast your full advancement comes in....
However, when you move up initial, your full advance timing will move up as well, so that is why you'd use a limiter plate or something that keeps the timing at full advance right where you want it. So if you want 36* full advanced, but your engine likes 17* initial, then you want your dizzy to allow only 19* of advancement. NOW, you have to decide what rpm you want it to reach full advancement....3500 rpm's???? 3000 rpm's???? Size of springs and weights in the dizzy will determine what rpm you get to fully advance timing.
This will ballpark you, but let the engine be the deciding factor as to what IT likes best. For me, if I'm running a vacuum advance, I don't push my initial up to the "start hard" point. In fact, even when I'm not using a vacuum advance I'll stay a little more away from that than most folks, but that's just me......
I'm sure others will chime in.... Hope this helps