Concerning the depth of the flagpole, you need to pay attention to the height and weight of it—(physics). Each flag pole comes with instructions of how deep the hole should be.
However, the depth of this hole should be that the sleeve of it sticks up two inches (just around) above ground level. The entire sleeve should be covered and down below, this should help allow you to finish it off with gravel at the bottom of the hole—this can help with drainage when it comes to rain.