Well on the subject of how much watage i see loads of places for arguement
but the best guide i saw took surface area and distance from the bottom of
the plants to set up a guide number.  Then divide this guide number by the
number of available florescent watts to give duration in hours.  Object though
is keep the number of hours between 8 and 12 or else you have to add other
factors that make the system far from linear on the ends.

Also keep in mind that all florescent bulbs do not produce the same quality 
quantity of light.  Therefore thse need to also be adjusted for.  Which brings
up another sytem of measurement.

instead of watts per hour the scale should be something like lumins per hour
at X frequency.  Fruthermore diferent plants require different amounts and
spectums of rays.  Examples my crypts can never survive with my swords because
if i give my swords enough light to flurish i'm burning my crypts out.

So in reality no scale is perfect only a starting point which can then be 
tuned for your particular situation.