I am using a T5 HO bulb (39 W) across the open top of my dragon's tank (no screen.) The hood for the bulb has a reflector.
My ?: How many inches down will the light go? Currently, his basking spot is 17 - 18 inches below the
lamp, but the floor of his tank is more like 22 inches away. I'm most concerned about the light reaching him when he is on the floor of his tank.
As long as he can get to a high UVB area that's enough. You don't need high levels of UVB throughout the tank. It's actually better to create a gradient so you have high UVB available and also areas with little to no UVB. They will move around to get what they need at any given time.
That said, the T5 with a reflector will generally reach quite a good distance. Optimal output is usually between about 12-14'' so I like to set the main basking area about that far from the bulb. You can put a branch or something like that a little closer to provide that option, and it's good to have shade areas and some basking areas a little further away in case he just wants heat.
That sounds good. It will take some constant little tweaks but after a while you'll get an idea for what he prefers. After general setup recommendations it really comes down to tweaking it for their individual preferences.