8"-10".
Be aware that brand-new UVB bulbs have a burn-in period of 50-100 hours wherein they can put out significantly higher levels than they are rated. When I first installed mine, I was getting UVI of 3 at 12" below the screen, which meant my perch needed to be lowered to 14" below screen to account for height of the cham.
After a week or so, I was able to raise the perch up to where he gets 3.0 on his back.
To be sure—and save money in the long run—a UVI meter is suggested/recommended, either a Solarmeter 6.5x or
DIY version. All T5 UVB bulbs are rated & warrantied for 1 year. I happen to have one that's over 2 years old and still going strong. Without the meter, I'd have replaced the bulb twice by now, so it's saved me the cost of 2 UVB bulbs (so far). The more enclosures (bulbs) one has, the faster/sooner the payback & savings.
The bulb in my chameleon enclosure is 15 months old and still at full strength as per meter.
One way to extend bulb life is to run it only 10 hrs. per day instead of 12. Very little UVB can penetrate the thicker atmosphere at dawn & dusk, so I've timed them to turn on 1 hr.
after "lights-on" and turn off 1 hr.
before "lights-out".