There is no way to determine the best adjustment for a particular bike excepting by use of an exhaust gas bench but one can determine a very satisfactory setting quite easily.
An engine runs best with a slightly richer than ideal air/fuel mixture. For normal gasoline blends, this is 14.7 parts of air to one part of fuel, by weight which is a good number to quote to impress the ladies at parties but otherwise just a number.
The most simple way to find an efficient setting is as follows:
1) Engine operating at normal operating temperature and other adjustments correct.
2) Maintain curb idle speed as specified but not necessary to be continually anal about the speed.
3) Adjust idle mixture to lean (turn idle mixture screw in) and out until one finds the idle mixture adjustment which requires the least throttle opening to maintain the correct idle speed. Don't be too picky about this as close is OK.
*NB. for those who use different terms, the idle mixture screw is the one which adjusts the amount of fuel delivery and is located in the screw well in the bottom front of the carb. and adjusted by use of a small slot screwdriver bit. The curb idle screw/idle speed is the one located on the left side lower which is adjusted by finger turning of the knurled knob.
4) Now take care and using ear, tachometer (or better a test tach which gives better indication of differences in speed), adjust the mixture screw slowly inward (clockwise) until the engine begins to stumble.
5) Turn mixture screw out (anticlockwise) slowly until the idle speed stops increasing.
6) Correct idle speed to desired curb idle.
7) Now slowly turn the mixture screw in (clockwise) until the idle speed (RPM) drops about 50 RPM.
8) Readjust idle speed if needed.
Here's the reason for the procedure:
An engine runs better because it produces more power with mixture slightly richer than the ideal air/fuel ratio. We find the ideal air/fuel adjustment by making a rough adjustment by setting idle speed, then adjusting air/fuel mixture to about where the engine runs best. Now we find confirm by leaning out until the engine begins to suffer from lean mixture because it will react more noticeably to an over-lean mix than to an over-rich one. We need to be able to tell that we are about right without going too rich.
Now, we find the real "sweet spot" which will be a bit rich, then lean a bit to find the ideal mixture by going a bit lean. Many people simply look for the idle mixture adjustment which gives the best running which is the shown by the smallest throttle opening.
If you lack an air/fuel or gas bench, this will give a very satisfactory idle mixture adjustment.
This all is much easier with a "thumb knob" type idle mixture screw or a 90 degree adjustment tool but a 1/4" hex drive slot driver bit will do the job. Buy a 1/4" Ratchet wrench as a driver and you're set.