transplanting

when do i put my rockwool starter cube into my expanded clay, when the tap root is showing? my plants have to half inch size leaves along with the small oval leaves it starts out with and the tap root has poked threw the bottom of the cube

You can do it now if you like, just hand water the hydroton once a day for the next couple of days, a small amount can touch the rockwool, but make sure not to soak it. If you do, he roots will take longer to grow, as it doesn't have to search for water. You could also kill it or give it root rot!