Trajectory Prediction with Particle Systems

Hi. I’ve been looking for a way to accurately present the trajectory of a projectile. The maths makes enough sense but I’ve been struggling to find a good way present it visually.

I found a video doing it perfectly, but I’m not too sure how they accomplished it. Could anybody share some insight.
Thanks.

Particle settings :
Gravity modifier = 1
Shape - rotation x, y, z = -60 , 0 , 0

Trajectory prediction by finding the startSpeed :
particle gameObject :
transform.position = startposition;
var distance = Vector3.Distance(targetposition, startposition);
var v = Mathf.Sqrt(distance / (Mathf.Sin(2 * 60 * Mathf.Deg2Rad) / 9.8f));
var main = fireParticles.main;
main.startSpeed = v;