ORIGINAL: Kweasel
The size of your tank and the engines abillity to draw fuel will determine how much the mixture will change throughout the flight. Peak out the needle pointing straight up with an empty tank, this is as lean as it should ever be. When the tank is full and the plane lever it will be a lot richer. If the difference is too much then use a smaller tank or smaller prop or smaller carb.
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Kweasel has nailed it. This is the only sure-fire way of determing your setting. You will be amazed at how rich your engine will run after you fill the tank and restart the engine. Resist the temptation to lean the mixture further. Learn to live with the early flight richness and deadsticks will mostly cease. Not only that, but your engines will last a LOT longer.
Ed Cregger