I would use a 22'' and set it over the basking zone next to the basking light. A T5 with reflector should sit about 12'' above the basking zone for optimal output (a little closer if it's shining through a mesh top which will intercept some of the output).
I'd set it like the top drawing so they're as close as possible. The idea is to generate the strongest UVB along with bright white light and high temps over the basking area to simulate natural sunlight. Then allow a gradient to cooler temps and lower UVB as you move away from the basking area so they can self regulate a bit.
Absolutely, as suggested, it's essential to provide a good balance of light. Making sure the
UVB & basking overlap on one end really helps give them what they need to be healthy!
I think your dragon will love the lighting arrangement.
Can I continue to use 100W Zoo Med Repti Basking Reptile Spot Lamp with the new UVA Reptisun 10.0? I figured the amount of UVA coming out of the bulb in negligible and wouldn't make a difference? Should I swap out the basking bulb?
The Repti Basking Spot Lamp is fine. I used those for years and they put out a nice bright light and plenty of heat (adjust the height so the heat gradient is correct). It's a good option to use in conjunction with the UVB tube.