If the engine likes 20+ initial timing, but can't crank with that much, pretty much the only option is to use manifold vacuum to give a lower initial, or some sort of start retard box.
If it does crank fine with 20+, then sure, limit the advance slots, and set the initial at 20+.
Or you can do what I did... one superlight spring, one heavier spring with loop. Initial timing is 16, but when the engine starts it immediately goes to the end of the loop which is 27 degrees idle timing. Slow advance to 36 after that (limited by rotor tower/plate).
Also in general, the engine vacuum at idle needs to be higher than the vacuum in the advance (if using manifold). If they're similar, then the timing will change with vacuum, which changes the vacuum, which changes the idle speed, which change....
It will never settle down and be steady. Similar issue if the step up springs in an Edelbrock are the same as the idle vacuum.
True about the unstable idle (same with centrifugal advance coming in too early). But it also works if the idle vacuum is
lower than the can (again, like mine). 8" at idle and the can does not move. At cruise (15") it's all in.
