With the motor not running, tighten the screw in as far as it will go (by hand, don't force it) then back it out 3 & 1/2 turns, this puts it pretty close (if it isn't - you may have other problems, like air leaks). Then start the motor and fine adjust the idle, in for up - out for down. (engine should be warmed up and w/o choke for this obv)
*btw - something simple like a hair-tie looped around the screw and the top of the carb, only just enough for steady, gentle pressure - will help prevent vibrations from backing out the idle set screw yet still allow for adjustment. I have one that serves the dual purpose of this and dampening the vibration in a loop of fuel line that was previously causing frothing issues.
Of course, now my hair is all unrestrained and crazy o_O
hope this helps