I hate to keep harping on this , but
Idle timing is nearly meaningless except as a means to an end.
The first time that Ignition timing is important is at Stall-rpm, and the Second is for Power timing. and Both times you gotta stay below detonation, with whatever fuel you are using.
Most times, to connect those two points, namely Stall and Power, you are gonna need a 2-stage curve, that is gonna begun at near 1000 rpm. But what the initial is chose to be, most often coincides with whatever it takes to generate that Stall-to Power timing line.
If you know the Stall-rpm, and you know what max Power-timing should be, you just connect those two, and the Idle Timing is then the fall out.
But, when yur done getting an Idle advance number, by fall-out, you still got to get away from a Tip-in sag, or a huge rpm-drop at N>D, which bangs the trans into gear.
So then, it can take a base-timing of anything from Zero Advance to say 12 degrees.
And in most cases it will take a two-stage curve, to not have too much timing at idle, which kills either the Transfer slot to Mixture screw fuel delivery ratio, or it bangs the trans.
The factory works that all out for you and gives the distributor a part number, that will have all the correct parts in it, and all you gotta do is consult the nice sticker they even put under the hood for you, and then everything will be peachy keen.
But here we are some 50 years later. Maybe the engine has been changed. maybe the distributor is not OEM, maybe the sticker is gone, whatever the case may be, and so, hey, we're on our own.
So what do you do?
Lemmee reiterate;
Idle-timing is as good as meaningless.
The short answer is set the Power-timing and just drive it, cuz at least it won't rattle to sudden-death.
So what is the max-timing number?
IDK, but I can tell you this, it is between 2 and maybe 3 degrees less than whatever causes the engine to detonate, on whatever octane gas is in the tank. and that's gonna be almost directly related to two things;
1) the cylinder pressure, and
2) the condition of the combustion chamber; be it temperate, shape, design, cleanliness, or the effing sparkplug sticking up into it.
If a man tells you 38* and you rattle the skirts into the oilpan, whose fault is that?
If a man tells you no, 34* is the maximum, and fuel economy drops to 10mpg, whose fault is that?
If another man says no, it's 30* but but at stall, the engine is gutless, whose fault is that?
IDK the magic number for your combo,
but I can almost guarantee you that if you set the Idle-Timing to whatever, it will be wrong EVERY where else, simply because of all the variables in play, between today and the day it left the factory.
I really am just trying to help.