There is no prescribed "setting". What that screw does is limit how far toward "slow" you can turn the throttle grip. The more toward the left you turn the screw, the more you can turn the throttle toward "slow" and vice-versa. Needless to say if you slow it down too far, it will stall.
+1. Some folks like to set it such that if they turn the throttle all the way down the motor will stall, others set it such that you can lower the throttle to the very low rpms, but not so low that it will stall. Mostly just preference. Screwing it in raises rpm, backing out lowers them. Here's a link for dialing in your carb too, in case you may need it.
