Dude, Im running my dwc using Lucas formula too, and I think you're burning the shit out of them if ur 8-16/gal.. That's for flowering, veg is 5-10/gal. For veg, I go with 5-10mL & ppm around 600-750 PH 5.5-6.2. Sometimes even with strong rooted clones in rockwool, some don't like it and the tips burn. I've found that starting them off at a lower ppm around 400-600 for the first few days, then taking ppm up to about 750, I'm seeing less signs of over ferting.
I'm no expert, and I just know what works for me. But is this your first run at this?