It depends on the local terrain features. Typically reading the mirage at the point where the bullet is the furthest above the line of sight is a good plan, as the wind speed tends to be higher the further above the ground you get. But if there is a cut in a tree line, or a ravine, or some other local quirk out there that is going to alter that significantly, then you'd probably be smart to at least check the mirage at those points as well.