The "Watts per Gallon Rule" is not a rule, it is a very inprecise guideline. It assumes that you are using a medium sized, regular shaped tank and much much more.
The real "rules" for light intensity are somewhat complex, hence this guideline was created, and it has been passed on, unqualified by it's limitations, until today we see it quoted as a rule.
Small tanks require more light then large tanks. If you have space to add lights and you want to give yourself a bit of flexibility when it comes to choosing plants, then add the extra lamp.
Once you have your lamps, anything between them and the surface of the water will absorb or reflect some of that light. Even a new drip tray will prevent some of your light getting to where it needs to go. In typical town water, a limescale builds up quite quickly and without extremely regular cleaning/replacement, the light drop off is quite marked quite quickly.
I build my own hoods from massive timber, don't use drip tays and don't have any troubles. If yours is made of mdf, hmmm, my thinking is that at best, a drip tray is only going to stop splashes, the air above and below the tray is going to be equally moist, thus a quality hood should be able to cope with the environment, but I also am well aware of how mdf can react to dampness. Difficult to be precise in the face of uncertainty. I'll PM CFC and ask, he knows much more than me about wood and that kind of thing.
Incidently, I used to live on the Surrey/Hampshire borders, if your water is anything like mine was there, then you will probably already be aware that it is much worse then "normal town water". Insanely hard and alkali. An RO unit was an absolute essential item, and a non metered water supply, very providential.
*** EDIT ***
From CFC...
MDF has no place anywhere where water and humidity is involved, when wet it takes on a texture like a dunked digestive biscuit and will litterally fall to pieces in a very short time. What i use is WBP (water boil proof) grade plyboard coated with a polyurathane varnish.
