loty, just as you noticed, this watts per gallon "concept" is not something you want to follow. too many other factors -- ballast, reflectors, bulb brand and type, distance from water level.
you're better off following rex's concept of using lux per square inch (of surface area) http://www.rexgrigg.com/mlt.html
if you really need some sort of guideline. downside of that is it is quite difficult to research how much lumens a certain bulb will produce, unless somebody out there has done testing.
if you really want an in-depth knowledge of lighting, read up on reefer forums. folks like Sanjay, The Grim Reefer, and Hahnmeister are very knowledgeable members at Reef Central. I believe PAR/PUR ratings are the most logical measurement at the moment
as for your tank, what did you mean by good light levels? just enough to keep plants alive? or do you want them growing at medium pace? or fast growth?