Distance from 4' T5HO

[zxyqu]

Id stick to the lower side of Jeremy's suggestion. A couple inches a week might be too much. I normally lower mine a little for summer and raise for winter. Granted this is less extreme that your purpose but I normally drop them at 1-2 chain links a week which is about 1/2 to 1 inch.

[Subrosa]

If you take your time adjusting plants like Catts that can take a fair amount of sun, the only threat T5HOs present is direct contact with the tube. My D. kingianum's leaves are about 3" away from a T5HO tube with a quality reflector on it, and those of my D. speciosum pedunculataum more like half that.