FC-3000

No matter what, you will need a little trial and error due to differences in environment that directly impact how many umol/s of photons a plant can handle in a 24 hour period. Late vege you'll want about 66% of what you intend to do in bloom. (this is based on 18hr vege cycle vs 12 hour bloom cycle -- you can see the suggestion for intensity is directly proportional to hours) In bloom i used my fc4800 anywhere from 16-24". I let it grow into light and will be 16-18" once fully stretched and 100% the whole time. These are powerful lights, the FC series. 24" at 66-75% will probably do you well. No matter what you need to observe and react to plants. Internode length will be your key focus in regard to light intensity... you want some length to develop but you don't want a stretchy, floppy plant. Becareful, as i said this is a power light. you could easily stunt plant growth with too much light and not see any other symptoms (if you do, it's WAY too much light, lol) Height should control your area of coverage coverage to optimize light at 100% power. A good height that pushes 'max dli', covers well into corners and possibly reduces a hotspot in middle. (bar style has less of a hotspot than any other option out there... you can see what i speak of my referencingn the PAR maps and comparingn the weakest/strongest spots) Make use of the dimmer beyond that. ** Some benefit to raising and increasing watts, too, but will be more wasteful and diminiishing returns on the benefit realized. The idea would be more photons spreadingn out, hitting walls and bouncing back at wider angles into lower portions of plant. E.g. you could run In an uncontrolled environment, you want to shoot for a DLI of 35-40ish. If you can push higher, just means you have a little better temp/RH or more ambient CO2 etc. All three are reletavistic factors for how much a plant can handle per day. Tightly controlled environment yo can push 50-55 DLI. This works for different hours of use... apples to apples.. 18hours and 55DLI will be roughly the same as 55DLI over 12 as far as plant growth. The "Max" dli will stay the same relative to environment. So, if you do map out what works for 18hours in your environment, it'll be even easier figuring it out for "12 hours"... and any point in future you'll have a good starting "ballpark" to quickly take care of finer resolution (small adjustments almost always needed).