Yeah, you got to lead those ducks :>).
I assume the cross hair is on a track above your flying object and is keyed over it. Obviously when you watch it in the preview window (what setting is your preview window?) it looks like the cross hair is right on, right? If you do a ram render of different sections of the scene is the cross hair still right on. How many key frames doe you have for the scene? I suspect you may have one at the beginning and one at the end. If the flying object proceeds at linear )constant) speed across the screen, that would work as long as your keyframe is set at linear and not smooth or one of the other settings.