I've done work on the engine and then readjusted idle on the Keihin carb many times.. the proceedure i use has never failed me. Only thing i can think of is maybe your carb could be cleaner. These butterfly carbs are extremely sensitive to the slightest bit of dirt or fuel varnish. The mixture screw should have a profound effect on idle.
Right now my mix-screw is set 1/2 (one half) turn out. This bike idles really well.. sometimes I come to a stop and cant hear the engine (but it' true, my ears are ringing), the idle is so smooth and low . I'm thinking the engine died or is about to die but it doesn't.
An inductive timing light works great to check it. When setting it i use a little battery powered stop lamp bulb with the engine off.. the lamp brightens (or does it dim?) just as the points open. The tiny screw and groove on the points that adjust them is just hard to manage.. a lot of care and patience is required.
If you can't get timing on the mark no matter what, it could just mean the point's body (where it contacts the cam) is worn down and can no longer reach the proper timing.
But if the trouble is just overshooting or undershooting the timing mark keep on trying .. move the points very slightly, tighten the screws, start it up and use the inductive light to check it... and do it again and again till it's right.