I'm 12 inches away from a 1kw in a parabolic reflector, i'm getting 10k at colas and 3/4 that at branch tops. Some colas are almost touching the rim of the parabolic.
I've read all your posts and what i found from your links was " Optimal is 7000-7500, or higher, lumens per square foot."
Optimal would mean - amount of weed produced per $1 of electricity used.
The most optimal light to own is a 600w HPS. If your space is 4x4 then you would get 21 ounces from a 600w HPS bulb but less and of poorer quality if you would put in 14 130w CFL's totalling 1820 watts. your going to pay 3 times the amount per gram of weed than you would by going HPS.
150w CFL bulb costs £20, 14 = £280, not including wiring them up with extensions and timers.
a 600w with ballast and reflector costs less than £120 with contacter and timer.
I still maintain, the answer to the original question isn't how much light do i need for my grow, it's How much weed do i need from my grow.