Light?

U can lower the light with the same setting or u put the light higher with a higher setting to dial in the ppfd. It’s the DLI that’s relevant. So 600 umol/m2/s ppfd over 18 hrs is the same as 900 umol/m2/s over 12 h period. DLI= (ppfd umol/m2/s x 3600s x hours of light ):1000000 Result is Mols per day.

With 600 you are totally fine as for autoflowers in the early flowering stage, a PPFD range of 600–700 µmol/m²/s is recommended to help the plants transition smoothly and begin robust bud development. This range can be increased in later flowering stages to 750-800. Good luck, growmie.

Ye you could do it but you should know that Your plant will need much more water and nutrients. Also i can Always recommand to buy undercanopy light since the results in scientific studies are showing remarkable effects.

hey 👋 Set it to the maximum bro 😉 no worries your plants will love it 👍

Use manufacturer's recommendations as a guide.

but really 640Watt ?? @ 50% = 320Watt and only a ppfd of 600? how high is your lamp ??? Us-Power-waste-style ? 1-3meters obove the plants ?!! I use 200-250W for 0,75m2 and have a PPFD of 600-1200 at the plant. With both lamps (LED 240W/HDL250W) i get near 1800 but temps rise then over 30°C - but the plants grow more in this 1-2h hot high energy phase.

The sun has a PPFD of arround 2000. So 600 is not to much. More light = more energy. (linear up to DLI of arround 40, from there you would need more Co2 to use the energy). Try your 75% or maximum and look if your plant likes it.

id say 900 is too strong for veg stage. you want 550 for late veg, 550 - 700 for early flower and then 700 - 800 for late flower. You can push it a bit higher if all other conditions are near optimal. if you want to put the light at 75%, just raise the light a bit and you can control the intensity that way.

DLI is what you need to focus on. Same DLI is fine... more or less might be a problem. Depends on hours of operation which wasn't given. If it's an auto on 18h, 600ppfd is all you need, assuming it is accurately calculated. Over 12h, 900PPFD is the same DLI. This is no coincidence. Regardless, you need to observe and adjust further from any suggested starting point. Better to get this dialed in while vege growth remeains -- node spacing of new growth is an excellent guide. If you figured it out for vege, then it's easy math for flower. 1:1 inversely proportional to change of hours of operation. 12 from 18 is 2/3rds, so 3/2 power for same DLI. 600 x 3/2 = 900. Easy peasy. Assumes all other relevant factors remain the same... e.g. hanging distance, temp, rh, co2 etc...