This reminds me of something I have heard online, and recently from a fish store employee who asked me directly - "what's your watts per gallon?" It's been a long-standing practice for people to judge their aquarium lighting by the nebulous value 'watts per gallon'. This is one of those Bad Ideas... almost as bad as the trusty old 'one inch of fish per gallon' rule-of-thumb that has undoubtedly ended the lives of countless pets over the years.
Watts-per-gallon has no specific bearing on how much light, or what type of light, a coral is bound to receive in your tank. It's just a rough estimate, and a poor one at that. Just because someone has X watts per gallon over his or her tank doesn't mean it's a proper setup. People like me with stock lighting on All-in-One aquariums like the Biocube are frequently advised not to attempt to keep SPS coral because our "watts per gallon" is too low. Don't get me wrong, I'd love to have more lighting and the stock PC bulbs on the BioCube are woefully inadequate for demanding species of corals and clams, but there's a reason why this is a silly measurement to use as a measure of how effectively your reef is lit.
Many people will tell you that light intensity diminishes with distance squared; and they are right. Some amount of emitted electromagnetic radiation (which we call Radiant Intensity) over a tank that is two feet deep is not going to have the same effect over a tank that is only ten inches deep, even if the volume of the tank is the same. The key here is how this emitted light is diminished and interpreted as Irradiance where we're really concerned about it- within the coral tissue itself. Irradiance is the measure of how much incident light hits a surface in watts per square meter.
Furthermore, not all light is created equal; corals (actually, symbiotic algae called zooxanthellae) prefer a specific spectrum of light to really thrive: between 550 and 700 nanometers is the sweet spot (peaks around the yellow bands). In the hobby, the spectral character of a bulb is described by it's "heat", and you'll see bulbs advertised as 6500K (yellow-ish) all the way up to 20000K (blue-ish). The reason people pick the blue light is because it typically makes the corals look much prettier due to the fluorescence from UV bands within these bulbs. But I digress...
Advanced reef keepers often own and/or share what is called a PAR meter. PAR stands for Photosynthetically Active Radiation, a measure of Lux, or Lumens per square meter. Lumens are a photometric unit of measurement, and the value is in context of specific spectral character; not normalized across the whole electromagnetic spectrum. Lux is very closely related to the aforementioned Irradiance, however.
PAR meters are used to measure the incident light that is most beneficial to photosynthetic growth; that means it is the closest true-to-life evaluation of how effective the lighting is at any point within the tank- wherever you place the sensor. This is why the tool is so valuable to hobbyists, as evidenced in this 2009 thread on the Michigan Reefer's forum. These meters are frequently traded around at club events since once you've used one, you don't need it again until your lighting changes.
Phew. Long story short, you can indeed keep SPS that are more demanding than many LPS corals in terms of lighting, but they should be kept higher in the tank if you are like me and have poor or moderate lighting. My Seriatopora sp and Montipora capricornis are doing splendidly and have tripled in size.... about 5 inches below the surface of the water. They don't look as good and their colors don't "pop" like they would under proper lighting, but full polyp extension and steady growth are the signs of a happy coral.
