My highest was at 17" when I flipped the cycle. That one got to 44". If you are looking for the most weed you can get and have room let them go some yet.
At the stage you are now, they should be eating the nutes up, they won't take very long to jump up in height. Just let them do what they are doing, they look as good as they could imho.
You got it buddy. So when you add water to reservoir I measure ph first. If say I am @ ph 6.0 then I would add in nutes at say ph 5.6 - 5.7 to counteract the reservoir ph. Always trying to be around 5.8 but off one tick either way is not an issue.
My reservoir is 80 liters and I am usually adding in 20 liters at a time to top off.