Assuming the t5 bulb is in a fixture with a reflector
12~ inches to basking surface.
If the fixture is mounted above the mesh
(assume around a 30% blockage of output +/- 5-10%) then 9~ inches from the basking site.
If the fixture doesn't have a reflector, you will essentially need to cut both of those numbers in half, as the reflector in most cases almost doubles the uv exposure.
This should give you a basking UVI level of around 3-5, which is what you want to aim for
-Brandon