The mesh tops on those tanks will intercept quite a bit of the UVB and deflect heat so I'd take that into account. With a 100w PowerSun, taking into account the mesh top, I'd say it should sit about 8'' or so above the main basking platform/area for best results. Without the mesh, 12'' is about right for those bulbs. These distances are for UVB exposure. As mentioned, you'll want to double check surface temps to make sure those are sufficient as well. The PowerSun may or may not be producing proper heat gradients at that distance.