rule is pretty much worthless IMO
There are so many more considerations wihen it comes lighting. Depth of tank, glass canopy, no glass canopy, distance from light to water surface, distance from coral to light, and most importantly what type of light you're using. 100watts of pc
is not the same intensity as 100w of MH
i'd be carefuly on what SPS
corals you decide to add to your tank with only 20watts of pc
lighting. You're corals wil be very close to you buls which will help. I'd def stay awary from some of the more light demanding ones such as acros...you may be okay with some montipora digita or some of the less light demanding types... but I'll let some of the experts chime in.