I have lived around and worked in Dallas most of my life(56years old).I have driven that same route 100s of times.A couple shots from the book depository was all it took.I'm sure most any body here could have made those shots.Oswald proved that the President of the United States could be hit.That should scare the [bleep] out of all of us.Just one nut case with a rifle can change the future.
To answer the question about the distance.At the most it would be slightly longer than 100 yards down hill maybe 20 degrees.You have probally made much tougher shots.

Ideas are far more powerful than guns, We dont let our people have guns. Why should we let them have ideas. "Joseph Stalin"

He who has braved youths dizzy heat dreads not the frost of age.