I've never used DNF, but I know others that have and I don't recall hearing anything necessarily bad about it. I wouldn't necessarily blindly follow the recommended dosing, however. 100ml for 20lt sounds a pretty high (that's one teaspoon per liter), but it all depends on the concentration. Use a TDS meter to make sure you aren't over-doing it.

Also, 6.5 PH is almost too high for hydro. Try to keep it just under 6.0 (5.8 or 5.9), since the PH range determines how much and which nutrients are absorbed. 5.8-5.9 is the sweet spot for hydro. This one change may increase your yield by itself.

Anyway, sounds like you're doing fine. Don't feel lonely, that's what this place is for! And post some pics of your setup...