The closer the better, the only factor is temperature. The light calculations say 0-12" the lumens are the same. Closer is more and better penetration and that is better. If you can control the temperature go for it. That's why I bitch so much about everybody monitoring the temperature at the top of the canopy. Did you catch my post about how I monitor the temperature in closets?

http://boards.cannabis.com/showthread.php?t=25057

If the plants get to hot like over 85f then the fan leaves will grow to large and block the airflow defeating the idea of the extra light penetration. You must balance the heat transfer with the light penetration for optional results.

Donā??t forget the airflow with the lights that close the air around the plant can heat up quickly and dry out the leaves and stocks.