I use a Medusa Temp controller to do the same thing. If my tank hits 84 degrees, it turns the halide off untill temperature drops to 82. It's hard on the bulb, to turn on and off all the time. So it's good to have some leeway.
I could use my ACII to do the same thing, but had the medusa sitting around, it uses X10 which I dont trust as much on Halides.
Chris