Here is how I make my spray, also I make enough spary for a liter you will have to convert for oz's. ( it's early and my brain is still asleep....). First I take the bottle of Neem oil and run Hot water over the bottle! Yes take the bottle and turn your hot water on and put your bottle of NEEM under it to warm up. I find this time of year that the Neem is very thick, this just makes pourning easier. Next I have a spearate cup for mixing, my PH water,a bottle of Dish Soap and a Teaspoon. Take the now warm Neem oil and measure out a half teaspoon of oil and put it into your mixing cup. Next measure out a quarter teaspoon of Dish Soap and add that to your mixing cup. Mix the soap and oil togeher ( try to get as much off the spoon as possible), now you can add the PH water,mix really well and then you can put it into your spray bottle. You may find that it all floats to the top.just keep mixing it together. This is normal
As far as the roots go, I have Never heard of or had any problems with Neem Oil hurting the roots. My last grow I had Fungus nats and Aphids...it was horrible. All I did was battle Ph and bug problems, anyways I would spray Neem oil right on the roots and remove the gnats that way. I was also growing in DWC. Good luck I hope that helps