Please study up on sniper fire and leading the target.
It is much easier to lead the target as it is heading away from you (in an almost straight direction) than it is to lead the target as it is heading towards you.
Oswald knew this. You do not, apparently.
That's right Bill, there are a number of reasons why Oswald's rifle rest boxes along with Oswald's prints were orientated down Elm Street.
1. As you say when you're high and the target is moving towards you it quickly drops out of your sights but when the Limo was going down Elm street the limo stays in the sights longer and considering that Elm street goes downhill that only adds to the available time.
In the following gif in frames 1 and 2 you can see that the Limo when coming down Houston doesn't stay in the sights for very long.
In frame 3 when the limo is passing right in front the Limo only is in the sights for a fraction of a second
But in frame 4 when the Limo is headed away the limo is in the sights for a longer time and is the more logical shot.
2. To fire down Houston puts you in full sunlight instead of in the relative shadows.
3. In the open Oswald would be shot by a hail of machine gun bullets.
4. To shoot directly down when Kennedy was right in front would mean that Oswald would need to be virtually standing to pull off that shot.
5. The angular velocity is highest when the target is moving in a line in front of you.
JohnM