The system needs a stable !!! ph with ferts.
I mix up the solutions in a 100 gallon tank, circulatin for about 1 hr, then check the ph and ppm. Adding ph up or down to a point of 5.2-5.4 with a ppm of 1200 over backgroud. At the end of the second hr, its usualy to the set point. Also use a circulation pump 1 hr pre fill to stablize the tank/s
Remember the larger the tank the easer it is to hit your target. With a small tank 5-20 gallons you must go slow. Or you'll be like a dog chasing his tail.
Now for the bottom line , watch your plants and see what they like, with your water and your system. Then you can fine ajust the ppm , to where they are happy.
Use good metered glass to make sure you add the same ph evey time. Never rush to adjust it ,,,,,sneek up on it.
If your going to adjust the ph on a hourly event of lighting I would suggest spending $5,000-$10,000 on a automatic system. Now thats just for the ph.
Omega.com has some great things to do that. National instruments , extech, to name a few.