Alex makes good points. The
wpg 'rule' is just a loose guideline, and is indeed based on
NO fluorescents. Thus when using
PC's or
VHO's, you can cheat a little bit. Good reflectors of course prevent loss of light, which helps even more.
(however no reflector actually 'doubles' your light to twice the rated wattage...its a legit claim though because without reflectors you're losing half the light from the top of the bulb, so any efficient reflector design will bring you back up to full wattage aimed into the tank)
as for my opinion on the threshold, I say 2wpg is when you should start injecting
CO2...but at 1.5wpg of
VHO or
PC lighting,
CO2 will have a positive effect on plant growth. Below 1.5wpg I don't think it's worth the hassle, though I've seen a few hobbyists who experimented with it, and did seem to have bushier growth.