just pour it on, dump the runoff, the rockwool will hold what it needs. When I have seedlings that small I do the same thing as you but I keep the seedling under my humidity dome with my clones until they show their next set of pot leaves (past the first tiny set you can see in your pics). They do great under the humidity dome with all that extra humidity, and whan I spray down the clones the seedlings get sprayed too and they are fine. Given your setup and the fact that your seedlings are still only in Rockwool , it would be pretty tough to overwater them. I say let it flow, but that is just my opinion. The one standing up looks great. It's tap root is probably showing through the bottom and ready to be planted.