All LEDs are different and their maker suggests different heights. However a plant your size would benefit from being at the 18-24inch marker. Also with the fungus gnats if you get those yellow paper traps that just stick in the ground it should catch any flying bugs, and no chemicals. I had bad bug problem, I did neem oil, spinosad, and hydrogen peroxide mixed with water in a spray re at bottle and it fixed it up good.
Happy healthy growing best of luck.