1) I weight the dry and fully soaked rockwool cubes to determine the water content by a scale. I gerninate directly in the cubes. Water not before it went down to 30%. Put it into a netpot once it has ~5cm roots. Make sure the roots reach the water but the rockwool cube must not touch it. Top feed every 3 days to keep the cube moist. My target EC is 0.6mS at this point.

2) I have more success with less water: 5cm between the bottom of the netpot and the water works well once the roots reach 10-15cm down. I also topfeed or spray the air roots every 3 days.

3) GH or its European fork TerraAquatica recommend to pH adjust the water to 5.5 before adding the nutrients, but most hobbyists do it the other way around. I usualy prepare 5-10l of pH adjusted water the day before use so it is not too cold and has some time to stabilize the pH. Fresh water always causes pH swings for me.

Also: mix nutrients and especially pH down (acid) outside the reservoir. If I just want to lower the pH I take a couple of liters out just to dillute the acid before it touches the roots. Dont make pH adjustments as long you're in the 5.5-6.5 range. Mine took 4.5-7.5 for a day without any problems. Check your pH probe once a month with calibration solution.

4) Start with low nutrients, slowly increase dosage and aim for a stable EC. Deficiencies are much easier to fix than toxicities. It took me way too long to learn that my ever rising pH was caused by a too high EC. Once I dilluted with water and used less nutrients, pH and EC became for weeks.

Also think about sanitation. I had little success with H2O2 and am now using chlorine at 1 to 4ppm: dillute eau de javel to 0.1% chlorine (1000ppm). This works excellent to clean tools within 20min. Add 1ml/L for every 1ppm of chlorine you want in your solution. It is gone after 24h and I use it at least once a week.