The presoak concept is controversial ime. I personally presoak in pH 4.0 water for at least 30 minutes, then do a secondary soak of mild nutrient solution at pH 5.8 before putting cuttings into the media. The idea is to buffer the pH in the wool to a reasonable level as I think off the shelf it's super basic, like 9+. The secondary soak just serves to precharge the media with a bit of a stabilized solution, and set the pH to the appropriate level.
Again, I'm not sure if that is at all needed, I imagine one could just get away with doing the secondary soak and be good. However, through all my research, pH 4.0 presoak seems to be the most common.
As 1989cheeseSOG said, you'll need to buffer your RO as well. Preferably with something of a strong base like potassium silicate (silica, eg Dyangro
Protekt), pH the solution down 6.5, add
calmag, add the rest of your nutrients, then final pH to 5.8 and let stabilize before use. RO itself has no buffering properties in it, so you will inevitably see massive pH swings and all kinds of weird plant issues. Depending on your nutrients, you may not need
calmag. Remember, wool is 100% inert, it has nothing to offer to the plant so you have to supplement accordingly.
Also, make sure you get your dryback cycles right. In veg, no more than 80% dryback is ideal meaning the media is still 80% saturated when the next fertigation cycle runs. In flower, you can do a bit more for crop steering, but that's a much more advanced topic in of itself. And finally, make sure you have some covers for the tops of your media. Algae loves rockwool and it can contribute to some pretty gnarly diseases like root rot, stem rot, botrytis, bugs, etc. Not to mention it robs your plant of nutrients and just looks gross.
