Light intensity is measured in PPFD (umol/m2/s).
We have made detailed intensity maps for our lamps that display what intensity is achieved at certain heights and how far the intensity reaches sideways before it starts to fade. I am attaching the PPFD-map for the Z2 model to this mail if you want to have a look at it. This information can also be found on our website.
There is not one exact single-digit to follow as there are a lot of factors that dictate how much light your plant can handle. A few of them are; temperature, humidity, how often you water your plant, and how much nutrients it's getting.
Generally, we recommend 100-300 PPFD for seedlings. Start at a lower intensity and increase it a little each day. Chocking the plant by suddenly giving it more than it can handle can result in tip burn, twisted leaves, and stunted growth.
Aim for 100-150 PPFD just when your seed is breaching the surface, and increase intensity in even increments slowly. Try to aim for 300 PPFD when your plant is around 2 inches tall and keep increasing slowly, up to around 400-500 PPFD during vegetative.
When it is time for flowering, aim for 600 PPFD or slightly more. Again, this is best done in increments. The distance will most likely be decreased automatically by the plant when it is growing taller and taller, towards the lamp.
Seedlings: 100-300 PPFD (30
= 191 PPFD center spot value) = 293 PPFD and 18
Vegetative: 300-500 PPFD (24
= 530 PPFD) = 600 PPFD)
flowering: 600 PPFD (16
You can try to carefully work your way from 16
to 12. As you can see, 12" will result in 871 PPFD, and this is absolutely doable but we only recommend it in the late stage of flowering. It also requires all other criteria to be met, as I mentioned before (humidity, temperature, etc.). Keep an eye on your plant, if it looks healthy and thriving, experiments with lowering the lamp an inch and give it a couple of days. If you see signs of stress, tip burn, or leaves curling, raise the light a couple of inches.
I hope this was helpful. Let me know if anything was unclear or if you have any other questions.