So since the T5 is much stronger than the t8, it's ideal distance would be 12-15 inches unobstructed. 8 inches would be the number if it were to be mounted ontop of the mesh.
I would look into perhaps changing how you have it setup. You also want it to be higher than the basking site (it looks about the same height in your picture, could just be the picture) that way it's not shining directly into their eyes. UV is necessary, but it can also be damaging. So if it's overhead, their eyes are protected.
Is that the Zoomed fixture?
-Brandon