An A/F gauge is useless for setting the idle mixture screws.
Assuming your carb has adequate idle fuel to the mixture screws and bypass air so no "nozzle drip" get the engine FULLY warmed up and well heat soaked.
Check the timing to make sure it's where it's supposed to be.
Set the curb idle speed.
Make sure the mixture screws are backed out sufficiently so the engine is a bit rich at idle. Slowly move them in one at a time and listen for a speed change or RPM drop in the engine. Stop, then back that screw out until the engine just speeds up and runs smooth. Move to the other screw and do the same thing. If the intake is fully divided you are pretty much done, if open under the carb you may need to balance the screws some.
The goal here is to obtain the best idle quality at the leanest settings. Once you have done this take a gander at your A/F gauge, this will be your base settings.....