WT, sorry to jump in un-introduced but this looks time-sensitive. I try to keep mine within an inch and have never had a problem, although I am using 2ft. lamps for application flexibility. Treat them like other fluoro tubes except the surface area for heat dissipation is much smaller than with T-8's or 12's, so the t-5's will fry your tender new growth in a matter of minutes if it contacts the lamp surface. (As opposed to several hours "grace period" with T-12's,)

I assume you already know there is a significant "sweet spot" in long fluoro tubes; the middle region giving you much greater plant response. It may be significant enough on a 4' 54w. lamp to require greater than 1" from the lamps, but I doubt it. My limiting factor is: '" How much will this sucker grow before I see it again?" If I put the lamps high enough so the plants don't grow into them, they're safe.

You're probably noticing that your 324w. is heating up your space a bit....flouro's DO put out some heat. It's just spread out over a larger area.

HTH, and thanks for all I've learned from you reading your posts. CP