I just bought a metal double walled last week so i could put a heater under it. How high is the water supposed to be in the pan?. everyday I have to shake it, lift up on it a bit to get some water into it. It seems to run down to about 1/4 deep. After you slide the outer case down around the well are you supposed to spin it so the tab goes into the slot?, or push it down all the way , then pull it up a bit so the water would be a bit deeper?.