Well, thanks a lot all of you guys for sharing your thoughts.
I have been experimenting a bit myself. my conclusions:
-the tip should be just-not flat. The smaller the curvature of the tip, the more it seems to travel.
-I raised the front of the table a little bit, so that it slopes toward the back. The top tends to travel further, but still manages to bounce all the way back to the front.
-the part that bounces against the rail is the upper ridge, not the part with the hole in it (to make it whistle).
I recently bought a third table and this one had the original top with it. It looks like the part that bounces against the guide rails is just in between the upper ridge and the middle part with the hole.
This one is much more bouncy, but the weight is almost half of the one I had made.