Prefecting spell reflect

If I am trying to reflect chaos bolt or gpyro do I do it when they finish the cast or when the projectile hits me.

POV, I am a Warrior

Ideally you want to throw up reflect at the last possible moment. That way you have the highest chance of actually reflecting it and not having them cancel the cast or hit you with something else.

What is that last possible moment is it when the projectile is on the way or when the cast ends?