hi there.. usually for flowering it's best to have between 40 and 60 watt per square meter.. here we're talking about watts drawn from the wall (most LED manufacturers say 600 and then there's i.e. a real 250w draw, so be careful of that).
If you have a quantum sensor, go for around 100 umol-m2-s for seedlings, 200 for cuttings, 200 to 400 for the veg-cycle, and then 400 to 850 umol-m2-s for flowering.. if you go higher than that you will have to substitute Co2 to the tent, so the plants can handle the intensity.
If you don't have a quantum sensor, this makes things a bit more tricky, but then, as Shagrath mentioned, put it up high enough, as seedlings are very sensitive and then lower the distance with time.
To get to the right flowering distance, i used to hold my under-arm (palm facing upward) under the light and looked at the reflection of the light on my skin. When it got lightly blinding (or i felt it getting warm), then i was around 850 PPFD (with my lights), so i knew this should be the maximum distance for blooming.
good luck and happy growing!