A question for your wisdom

Hello!!

I wasn't quite sure where to put this because it both involves movies AND guns!! But, anyway, I'm a screenwriter, more of a wannabe at the moment because I haven't sold anything yet. But anyway... I've just started writing a gangster script and I just want to make sure I'm doing things right.

The scene I'm working on now involves a sniper shot from the roof top of one building across the street to take out man in an office. My question being, can I lay the scope cross hairs directly on the man's head? Or will I have to show my assassin working the elevation knobs. A shot across a downtown New York sized street (generic unnamed city), maybe 15 - 20 stories up.

thanks a lot!