Well I just realized that you weren't growing fully in Rockwool like I do. But to answer your question, I start my seeds in 1" rockwool cubes. Once roots start to fully come out of the bottom, I switch them to 6" rockwool cubes and add my drippers.

I have a 8 gallon bucket that I use as my main res that feeds my 6" cubes. WHen I check the PH of my res I will also take a sample of the water that is exiting 6" cubes. It's easy for the way my set up is. The 6" cubes sit on top of a 6" x 36" tray. The drippers feed the rockwool. The execess water comes out of the bottom of the rockwool and runs down the tray to exit pipes that lead back to the res.

So I take a sample of this exit run off water. Depending on the lifestage and what I'm feeding the run off can be off as much as .2 for every feeding. This seems like a lot and it is. But I run my drippers 15min for every 3 hours. So after 3 feedings or so I will adjust the PH of the Res (by this time it has also dropped) back to 5.8-5.9. The 15min that the drippers run easily bring the run off of the Rockwool back up to the correct PH.

In a nut shell, the PH of my rockwool starts at 5.8-5.9 and throughout the day drops to 5.2-5.3. Then I reset by uping the PH of the res and it starts over. I have not had any PH related problems as of yet. I know a lot of people do not like Rockwool saying it gets too wet and promotes root rot. I have never noticed this. Rockwool holds a lot of O2. Yes it stays moist, but this hasn't been a problem for me.