Photobio mx680

16-20" maybe slightly further away if frame is smaller than i think... for 18hours you'll want 66-70% power at that hieight, then ove 12/12 cycle you'll want 100% power. Some trial and error still needed, but this will be close enough to work out the rest with a week or 2 of observation. whateve works for 18hous you need 150% of that intensity for 12 hours -- it is proportional. (read up on Daily Light Integral - DLI) - A lower light focuses more light on a smaller area (a viable option for affecting intensity relative to canopy) IF the frame is 42-46" x 42-46" then 18" is going to be great distribution of light. if diodes are more tighly packed in smaller area, you may need a bit more height from canopy. it'll f unction more like a QB if too tightly packed, which is still a great option, fwiw. you can give moe light at higher distances on a 18/6 cycle, if you want... the extra height will reduce hotspots udner the light and more light will hit lowe areas of plant bouncing off walls at more obtuse angles... but is less efficient too... for 1 light, small cost difference. I only mention cause not all lights have dimmers. e.g. mine only does 100% then 80% then 60%... so have to use a similar 650w light with a little more umol/s than this one at about 28" and 80% over 16/8 schedule... 18/6 is too much at thet height and power. When bloom hits, i know i can get away with 18" and 100% power over 12 hours. so, dimmer%, height, and how long it runs can all be used to fine-tune how much light you give a plant... this is based on atmospheric CO2, you can probably give 25-33% more umol/s per sq ft if you are supplementing co2... but you better also tightly control environment otherwise too, in order to achieve those increases.