Hello Dale,

"That said, how far above the seed starting tray and the ensuing seedlings should my light be?"

The primary guideline here is you don't want to overheat or burn your seedlings with exposure to heat from the hot T5HO bulbs. HO stands for High Output and that means the bulbs are hot. I would say your T5HO bulbs should be at least a foot above the seedlings. But you can use your hand as a temperature sensor and tell if your T5HO bulbs are too close.

There is a lot to know about gardening under lights. I started about 18 years ago when the "in" fluorescent bulbs were T8s, and you got enough light from them by overdriving them. I still use overdriven T8s because I have a significant investment in them. I suggest you get one or more books on the subject of Gardening Under Lights (that is a clickable link)

My favorite book on the subject is the one by Leslie Halleck. But it doesn't hurt to have several books on the subject. One of my hobbies is breeding zinnias and my old overdriven T8 fluorescent lights let me pursue my hobby during the Winter. I have been growing zinnias under lights for several months this Winter. And I am still learning stuff.

ZM (not associated with any product or vendor mentioned or linked)