Does it have a light that goes on to show when it's heating?
i Pretty much ignore the settings on the heater and go by what the thermometer hanging in my tank says. If my heater light comes on when i want my tank temperaure lower, i just turn the dial on the heater down slightly until the light goes off. The next time i notice that the light is on, i check the thermometer again, and if it still isn't as low as i want it, i again turn the dial down slightly, just until the instant the heater light goes off. I keepdoing that until the temperature has lowered to where i want it to be. Same goes for raising the temperature, but it's just in reverse. When i notice the heater light is off, i check tehe thermometer and if it's not warm enoigh, i turn the dial up slightly until the heater light comes on' and repeat that every time i see the light go off until it reaches the right temperature. I takes some fiddling around and tweaking, but eventually it should stabilize.