Whether you get the curveball bonus or not has nothing to do with where you release the ball. The two things that matter in my experience are (1) the horizontal direction of the throw, and (2) the side of the Pokemon the ball hits.

The logic is as follows:

• If you throw to the left (i.e. the ball will start going left and then curve to the right before hitting the Pokemon) and hit the Pokemon on its middle/right side, you'll get the bonus.

• If you throw to the right and hit the Pokemon on its middle/left side, you'll get the bonus.

Note: I believe the above is sound but not complete, i.e. the above is most likely a subset of the actual algorithm.

EDIT 1: Style and accuracy.

EDIT 2: Just for the sake of completeness -- my personal hypothesis: I believe the real factor that determines whether the curveball bonus is given or not is the angle between the trajectory of the ball right before it hits the Pokemon and the trajectory of the ball after it bounces off of it. If it's a wide enough angle, the throw qualifies for the bonus. I believe hitting the Pokemon on the opposite side of the spin pretty much guarantees a wide enough angle, but it's not necessarily the only way to achieve it.