I use it centered over the normal plastic tub top. It's actually on a clamp on the window sill above the tub right now, the bulb is nearly 2ft from the top and about half power giving me 83-84f ambient/cool side temps. I'm not really using the uth much my blood likes the cooler temps (he's almost never on the warm side and moves the hide off of the uth).

The stand is just extra it doesn't come with a dimmer: http://www.petsmart.com/product/inde...AvailInUS%2FNo

For such a tall tub you will probably need the heat source closer to the top of the tub. The lamp stand is adjustable which makes it nice.