Just transplanted some babies today and I just keep the water high enough to reach the Rockwood until they reach down below the net cup. That way the RW stays moist. I don't know if this is best practice but it has worked for me. Will be curious to see some other responses.
Im assuming if one root is popping out of the bottom, theres sure to be another coming soon.. it was a tiny tiny root. my first 5 finger leaves are now just coming up so its still a baby. Will keeping the bottom of the rockwool submerged be enough to feed the entire plant if only its just beginning to show small roots? Will the water wick up?
From what I have seen if any part of the RW is touching water it saturates it. The only thing I have worried about was lowering the water level before I get any issues like stem rot. However, when I have a full on flower process going the RW is in there for the life of the plant and no stem issues but the water is lower by then (near the bottom of the net pot usually).
Also, in my experience once a few roots pop through you aren't far from having them exponentially grow and before you know it you have a full on root ball going.
I don't think you are supposed to leave it on there, should come off before you plant or germinate. Seems to me it would impede the roots coming out the sides..