Or 0, or even 0.01 should be fine.
To make the caster utterly undetectable, I give it Invulnerable, Locust, and Ghost as abilities (to make it undamagable, untargetable, and non-colliding, respectively). Then I set the model, soundset, and shadow to None. Then I set the movement speed and turn radius to zero, so that it doesn't waste time turning before casting a projectile-using spell, and lastly change both of the spell-backswing-related Art fields so that there's no animation delay before the spell is fired. If you like, you can give it a negative regeneration rate, so that it dies on its own, but it's safer at any rate to kill and remove it with the trigger anyway.
This is the best way to create a caster that isn't visible, doesn't interrupt gameplay, and can perfectly execute commands given by trigger instantly.