Looking at your drawing again, they would probably reach longer if they used one of these.
But still as Galadriel's drawing and my simulation pointed out. The catapult at the back wouldn't reach the furthest into the lines. The height isn't such a big advantage except if the stones were thrown away very HORIZONTALLY and which much greater force. (Like a bullet)
But these are thrown high up in the air and therefore doesn't reach that far. (See simulation for more detail) Also the movie itself confirms the simulation and the drawing so...game over buddy.
http://www.youtube.com/watch?v=SAW2LxJRLqM