Trajectory is the wrong way to think about it.
The disc is a wing, airflow is what keeps it aloft and thus, keeps it flying forward. As long as there is enough airflow, the disc will produce enough lift to stay aloft.
Of course, the disc also has to stay stable which is where spin and release angle come into play as well.
The short answer, throw it level with the correct release angle and the disc will take care of the rest.