It's just the idle mixture setting. Setting to overly rich has no point what so ever and will only foul your plugs. If you don't have any kind of WB-lambda or gas analyzer at hand, take it counterclockwise in small increments till it idles badly, then about 1/8 back and it's fine. Remember, having the tuning hole unblocked will affact the mixture. Also, having the allen key in, will affect the mixture.
About the ignition timing marks, it's the second mark that is zero and they go in increments of 4.