Recommended PPFD levels for cannabis:
75-150 µmol/m²/s for cuttings,
100-300 µmol/m²/s for seedlings.
300-600 µmol/m²/s for the vegetative stage
over 600 µmol/m²/s in the reproductive phase
You could go as high as 800-900 umol/m²/s in flower if you got everything dialed in.
Just use your phone there's apps called Photone or PPFD Meter to measure your light outputs.
Pretty much it's like roasting a chicken, you wanna get that sh*t done and taste delicious, but not burn it.
Joke's aside, try see what values you're geting at 50% and at 100% @ 20" height would be a starting point, go from there.
Just keep in mind the higher you hang your light, the more photons are reflected by the wall instead of hitting your plants directly.
But you don't wanna go too crazy cause too much light can burn your plants without radiating alot of heat.