The best A/F for mileage will take a while, because the amount of vacuum advance must be (at least) roughed out before trying a mixture test.
Don't be surprised if the best mileage comes from +15° on advance and 16:1 (not 14.7:1) ratio.
Yes this. I run the cruise mixture a lot leaner than most guys it seems on a moderately tuned engine such as a 302 GMC or 261 Chervy with about 9-1 compression and a cam with 215 degrees duration at .050 lift.I jet down the mains until the engine hesitates at slight acceleration in high gear at about 45 mph.Then jet up a touch. Running about 20 crankshaft degrees of vacuum advance and about 20 degrees of mechanical at that road speed.Heated intake with a 180 degree thermostat. The plug porcelain will generally be dull white.The light tan color is usually too rich at a light cruise engine load.
There's many variables here depending on engine tune and gearing ,the carb can be still running off the idle transition circuit at 1600 rpm with the main jets coming online around 1700 rpm.Engine temperature,heated intakes,fuel octane,compression and combustion chamber design and quench all affect the jetting.And messing with the main jets affects full throttle mixture as well .And few engines may have better fuel mileage with a slightly richer jetting that I aim for.
Trial and error is the only way. And for sure an A/F meter is very useful.
