minimum

I thought I met the conditions, but can someone enlighten me on what I need to do :) Please and Thank you! :D

Can somebody explain the watts/sqft number people are using on these forums? Or point me in the right direction for the answer? I see many people saying 35-40 watts/sqft is optimal, but what's the math/science behind that? Wouldn't something like that only be reliable if you are standardizing...