There’s no way Oswald could have known that! If you’re prepping for a snipe, you practice in intervals! 20/50...etc.
A. The presidential route was published in newspapers. When Oswald set up his sniper’s nest, he knew how far his best shot was going to be.
B. Everybody overestimates their own abilities. It doesn’t seem like something that would stop Oswald.
C. The actual shot was close and doable by a poor shooter.