The idle mixture screws are trims to the real low speed circuit which is the transfer ports.
If the transfer ports are flowing too much, then the trims will be small, and vice versa. It's just a balancing act between them. If the engine will not idle with the T-ports in the ballpark, then you add idle air from a bypass( usually incorporated in the PCV line, or add ignition timing until it does.
Both of these are also balancing acts.
Too much bypass air leads to tip-in hesitations, and really really stinky burns-your-eyeballs stinky, exhaust; that cannot be trimmed with the mixture screws.
And too much idle-timing CAN lead to low-speed/low-rpm detonation; usually with a manual trans or near stock TC.
Also when advancing thechit out of it, the idle speed may rise so high that you want to back out the curb-idle speed screw........ and thus mess up the T-port sync...... and introduce hesitations,and an unstable idle crappy idle.
So the primary part of the solution is to get the T-port sync established and then LEAVE THE SPEED SCREW ALONE.Then ensure your streeter has a properly plumbed and working PCV.That your streeter has a Vcan hooked to the spark-port and is dead at idle. And for all typical street cams even up to about 276* intake duration cam,that the secondaries are closed up tight but not sticking- both valves. Then set the front mixture screws in the middle of their sweetspot, about 3/4 to 1 turn for Holleys, and shut off the rear screws.For a SBM, set the timing to about 14*.......
This is your starting point.
Ok so what is the starting point T-port exposure? IDK; but I have an opinion. The exposure under the primary valves should be square to slightly taller than wide, and the smaller the cam or the higher the Scr or the closer you are to spot-on, the shorter the slot can be. Your engine will tell you by the settings of the trims.
Now, if this is a 360 with a 276 or less cam, and matching compression ratio,and you have yours already set-up this way, then it should already be idling nearly perfect. How can I say that? Cuz that's what I am driving.
But if not, then NOW we can try to figure out the why of it.
And here are the principle players;
1) low and/or unstable wet fuel level. this can include air blowing over the bowl-vents
2) unstable ignition timing. This can include the Vcan dicking around.
3) Vacuum leaks, including into the valley, and the P/B booster, or anything plumbed to the intake
4) unstable engine temperature, including an electric fan cutting in and out
5) uneven cylinder pressure; this can be from an entire subset of issues including;
..............2a)leakage from rings or valves
..............2b) bad lifter preload or valve lash
..............2c) bad cam
............. 2d) bad valve gear
............. 2e) Bad valve timing.
..............2f) bad headgasket.
..............2g) fuel-washed cylinders I guess.
6) hot and/or overheated sparkplugs
7) Not likely but; an overfilled crankcase
8) lemmee think, I think I got it all.