They can't precisely tell you what settings to use because it can vary significantly based on the brand/type of fuel and the weather conditions in which you're running. I would guess that the factory settings are usually generous with the amount of fuel that going into the engine. I don't particularly feel that's good for the engine because it cools it too much. I would suggest leaning it right away if the engine is exhibiting signs of running too rich. If the performance is good and the engine temperature is above 150°F, then I would say you're close enough the right mixture setting if you're driving it cautiously. It's a good idea to give the engine about 5 tanks of fuel before you start pushing it harder.