[TabMorte]

I think it's very subjective.

To me Low would be one or less WPG
Medium would be over one but less then three
High to me is over 3

That said it depends a lot also on the dimentions of your tank. I have yet to meet a plant I can't grow in my 10G by just plunking it down cause it's so shallow everything gets so much of the lightly. Where as on my 30 which is so much deeper I really have to think about where everything goes because the plants are further from the light and tall enough that the larger ones shade the smaller ones.