Light distance and intensity.

while it's nice to have a 0-100 dimmer, at some point you do lose integrity of spectrum produced. get a smaller light for early stages -- save even more money and maintain advertised spectrum tailored for photosynthesis. or, stick to 50% + and raise light to 24 inches, may need to watch and react to dial it in. i used the fc4800 at 24-36" and 66 % -- slowly lower as the seedlings mature... your vege power will be oughly 66% over 18 hours vs 1hours for bloom... easy adjustment at flip.. simply bump light to 100% and maintain same height. looks like you grow autoflowers, so you can increase height and increase coverage area and use at 100%. hours of use is a relative factor -- adjust 1:1 in aproportional way if all other factors remain the same, like distance from canopy. figure 50-60umol/s per sq foot will do you well.. use spec sheet to guesstimate area of coverage for 18hours/day using this info. adjust height of light to best cover this area (a cheap lux meter helps measure various locations to even it out)... 3'x4' might be doable. any smaller area will require some dimming at that height. distance from canopy is an option to adjust such things, but it's better to use height to adjust area of coverage / even out coverage as best you can. a few inches either way is no problem, of course. only if it causes me to over-dim will i consider changing height. Seedlings are definitely one of those situations. so this is more for when things are rolling. with seedlings don't be too put off by what i said about spectrum integrity... i use a xs1500 at 10-25% on viyyomhd all the time, but with seedlings i don't need to drop below 50% at a decent height for coverage. when you use a large light for small plants, it's a little sub-optimal. err on the side of slightly weak light and adjust to stretching quicly as needed. it's easy to stunt seedlings with a big light. can always 'fix' a slightly stretched seedling with xplant depth vs a stunted one that just takes time. if it ever grows out of it.