Fungus Gnat

These things can be incredibly destructive; I grow rare & heirloom fragrant plants & watched helplessly while the larvae selectively slagged my most irreplaceable plants. I had applied predatory nematodes as a preventative ; since I did not lose everything, they may have helped. I applied a BT drench yesterday,& am applying a diatomaceous earth barrier today;then I will spray Neem oil everywhere,(& pray).
I was told predatory mites(Hypoaspis sp.??) were better than nematodes at fungus gnat control,& will try that too. Can anyone tell me how much diatomacous earth I need to mix into my potting soil to make it hostile to gnat larvae?? Not just as a barrier; I'd like to incorporate enough into the soil to permanently check any future invasions.