This approach will lead to inaccurate pH measurements, as when the plant grows, the pH inside the cube raises too. By only replacing tiny amounts of nutrient at a time you are simply compounding that problem! But by running the pump for 15 minutes at a time it allows the fresh nutrient to permeate through every part of the cube, pushing out the old solution. You should see it dripping out from different points across the base.
When plants are first installed in the rock-wool the roots will be minuscule inside the peat pellet, so we only run the pump once a day for 15 minutes in the morning, as we want the cube to dry out slightly over the day to stimulate root growth. As soon as you see the first roots come out of the base of the rockwool cube, run the pump twice a day in the morning and early afternoon. When these first protruding roots have withered and died back and you see the following reactive cluster leaping out of the base you know the system has worked. Keep the feed strength at 1.2 EC, pH at 5.5 and feed this 4 times a day, 15 minutes each time. Have the last feed about 2 hours before lights out.
As the plants grow you will see the most outrageous root system emerge. Allow the roots at the bottom of the cube to die repeatedly and hold your nerve… Keep them on the screen for about another week, until the cube is literally carpeted in fresh vibrant roots. Increase the number of feeds a day gradually over the week, judging how long it takes for the cubes to feel lighter. By the time you move the plants on you will need to feed about 8 times per day (in an 18 hour day), as there are literally too many roots to fit in the cube and it will be thirsty and dry out fast; this is why the extra water-retentive properties of the peat pellet works in our favour now, by holding a little extra moisture for longer right at the core.
Peek down the side of the plastic wrapper and you should see a mass of pure healthy white roots, looking like the cube in the picture. The plants are now ready to transplant into their final home. Enjoy your massive yield!