You do not lower idle by changing the mixture. You lower the idle by... lowering the idle!! Use the trim on your transmitter, and if it is already bottomed out, then readjust the linkage to get a lower idle speed.

Once you have set your needle valve for optimum high speed operation, you leave it alone. You want the high speed (full, wide open throttle) to be set just on the rich side of lean to provide lubrication, and leave it alone for that day. It changes daily due to weather conditions, pressure, humidity, and so on, so set it before flying the first time and leave it. Adjust your idle, as I said, by the trim lever on your transmitter or by readjusting the throttle linkage.