If you're using conventional T-12 fluorescents (the larger diameter tubes) the usual distance is around 8". If using T-8 or T-5 tubes, the distance can be increased. Your plants will let you know if they're getting too little or too much light, and you might need to adjust the height of the fixture a little, or maybe just move the plants around on the shelf. Some varieties like more light, others less. You don't really know until you see how they respond in your conditions.

Mark

Thanks....I have a T12 and a T8, 48". Guess I willl have to compromise and experiment until I find the sweet spot.