From my experience, especially with the Mars IIs, is it can be very strain-dependent. I ran a pair of Dinafem WWs a couple years back and with the MarsII, I had the light less than 1ft (30cm-ish), from the plants and they didn't bleach one bit.. I just grew some very tasty NLxBB, and even with the lights approximately 1.5ft above the canopy, the plants were still stressed.. You can tell the stress by the leaves curling up; generally indicative of heat stress (lights being close), and as I was told and reaffirmed, MarsIIs use UV LEDs which generate more heat than the typical red/blue/white.. I do fully agree; that light needs to come down a bit to fully utilize it's power.. Tough to get a good dimension, but I'd start 18" (approx. 40-50cm), and see if they respond well to it.. You'll know within a day or 2 if it's too close.. Trust me on that one..