Add small offset to Y rotation?

I have created a projectile that gets destroyed after traveling a distance. When it gets destroyed, I want to spawn 5 new projectiles that each have a small offset on their Y rotation. I created this method:

 void split()
 {
     Quaternion newRotation = transform.rotation;

     newRotation.y -= 0.3f;

     for (int i = 1; i <= 5; i++)
     {
         spawnSplit(newRotation, transform.position);
         newRotation.y += 0.15f;
     }

     StartCoroutine(DestroyProjectile(0f));
 }

This works kind of right. However, this doesn't work like I want. When the original projectiles' Y rotation is around 0, all the spawned projectiles will only have a small offset, but when the original projectiles' Y rotation is around 1, the spawned projectiles will have a big offset.

How can I make sure all spawned projectiles will have consistent offsets? Thanks for reading :)