Germination in rockwool, when to turn on the light

As the title states. When do I go to 18/6, is it when the seed has popped through the top of the rockwool or after a couple days and turn on the light? I removed a piece of rockwool that I used to cover the hole and could see the tap root, so what should I do? This is for 20 seeds.

I turn my light on around 24 hours after I first plant the seed. Everyone does it a little different

I put mine under the light when I remove the humidity dome. I tend to poach things if I turn it on before the dome is removed.