Instead of visualizing it as watts per gallon try visualizing it as 1 MH light for every 2 feet of tank. Now the watts of the MH light would be depending on the "depth" of the tank. Now my tank is a 120 gallon tank. The tank is the same length of a 55 gallon which is 48" long but its 24" wide and i think 24" deep. Something close to that, but anyway, i have 2mh lights on it @ 250w each. Only because of the tank depth, are the lights 250 watts, also there are 55watt pc's on them also but only for the actinics, thats it, no other reason. You can have 6, 110w VHO's on your tank on a 100 gallon tank which would be 6.6 watts per gallon, but the light thats being generated from your lights would only benifit the "upper" part of your tank or the corals near the top most of the tank closest to the lights. Reason being that the VHO's cannot "push" the light down to the bottom of the tank the way MH lights would. So try not to take the watts per gallon theory to heart just try to figure out the best way to present your light to the corals in your tank. All of this lighting is also depending on the corals in your tank. Hell you can get mushrooms to stay alive with just regular NO bulbs. Its all about placing the light at the desired levels to the coral.
Almost forgot that I only use Metal Halide lights. IF you cannot afford metal halide lights for your reef tank, do not bother putting stuff in there that has a high light requirement. If your serious about a reef tank with funky lookin stuff in it such as clams and sps corals, dont even bother playing with VHO lights, go straight to MH lights and then you never have to worry about the lighting in the tank again.
