They need 2k to 4k foot candles. I'd say about 8" from the bulbs in the center of the light and 6" to 4" near the ends. You really should get a light meter. They're about $80 bucks or so and very helpful growing under lights.

I've never used it before, but Hydrofarm has a ~$20 or so light meter that I've seen at many hydroponic stores...

http://www.hydrofarm.com/pb_detail.php?itemid=3087

Amazon carries it too..

(I have no relation to hydrofarm, I just own some other hydrofarm products and enjoy using them).

Here is a link that might be useful: Hydrofarm Light Meter