Your plants can get very close (as long as they are not touching the plants) to fluorescent grow lights because they don’t become nearly as hot as other types of lighting.
For seedlings, the T5 fluorescent grow light distance should be a little bit farther away. To begin with, I would hang the light 6 to 10 inches away from the seedlings. Whether you are using a standard fluorescent light bulb or a huge T5 fixture, use the same distances.
For the vegetative and flowering phases, hang it 4 to 6 inches above the tallest plant.