This will give you the ideal watt hours of fluorescent lighting that you
need. Divide this number by 11 and you now have the approximate total
wattage of lights you need.

Hmmm... This seems to be a problematic formula. Example: 6' x 2' x 2' tank.
Surface area: 1728 square inches
Distance from gravel to light source (pendant light): 30"

The formula is for flourescent lights. It doesn't apply for MH lights.

But yeah, this is like the "watts per gallon" rule. It's only a rule of thumb.
It only takes tank volume into account. It doesn't take into account that
tank depth can matter more than the surface area. Less than 2 wpg may be fine
for a tank that's only 12" deep, but not for a tank that's 24" deep.


Leigh

http://www.fortunecity.com/lavender/halloween/881/