Simply use watts/liters.
..5 w/liter to 1w/liter.
You can have more or less and do fine also, but this is a good general
range and offers a fair amount of wiggle room to account for the large
Lighting is the biggest variable we have, the bulbs that are new in a
tank vs the bulbs that are 4 years old are certainly not the same. Many
things come into play.
I think overall, folks addressing the CO2/nutrients will get the most
out of their lighting, no matter what they have as long as they are in
the LCP range or higher.
I've found that larger tanks do quite well with less light also, I
think this is due to scattering and better angels of light strike on
the leaves, at a short shallow distance, the light only comes from one
very intense angle. These shallow tanks tend to have one buln, whereas
the large tanks have several, this gives a nicer spread of light.
I like HQI's but they do not produce the nice even spread of FL's
So I use both and this gives me the best of both.
But that's very high light, so I turn on the HQI's for 3-4 hours and
then the PC's for 10 hours at low light.
Good write up John BTW on your site.