i started with mine wet and I could only flood every few days in veg and very day at lights on in flower.
the 2nd time i tired this but found out they dry out faster if you start dry.
weight and how long it takes to get light. I mean the cubes actual weight. you have to learn to jusdge that. if your plants get light enough as in they drink enough water and it dries up fast enough then you can feed. not before then.
i would say the number one problem is overwatering in rockwool. it will grow and look ok until you back off then wow the growth starts. howver i have done overwater and underwater.
judge by weight and sewt your pumps accordingly. that is all I can say. 15 minutes seems like a long time. I have always read you should only flood for how long it takes to fill the tray. that way you can feed as many times as you want. rockwool loves the actual process of drying. it pulls all kinds of air to the roots. that is what it is all about. getting air to those roots.
as the previous poster stateds dont ever let it dry out though. all kinds of bad thngs go wrong then. lightly moist all over is good. also how high do you flood? I would only flood 1-2 inchs up the cune if I were you.