My personal opinion is that it's a balance of nutrients and light. For example, if you have low nutrients you might be able to get away with lower light, higher nutrients higher light and sometimes possibly vise versa. Some tanks like mine will show low nutrients in a test but that's only cause I have so much stuff taking up the nutrients so I feel like my tank likes more light. When I've gone with ulns's I felt like the tank could get away with less light and when I started adding light it would burn things.
Like anything in this hobby it's all a balance and more than that it's about finding something that works and sticking with it. Look at the tank at aqua terra. 1 kessil hung extremely high has kept a lot of things alive for a while now. That can't be putting out more than maybe 30-50 par on the edges but it works.
