I think it’s about 10%, this tank is in my bedroom so I need them to be dim for sleep. It’s my understanding that ‘moonlights’ aren’t entirely necessary but can make viewing of the nighttime activities easier as well as provide a ‘moon cycle’ for species like RFA which are believed to possibly time their spawnings around equinoxes or the moon cycle (although nobody really knows). It’s mostly just a fun thing for the aquarist, I don’t think the reef cares that much. Of course, if you were running something like an octo/shark tank where they are active more at night that is a different story.