Honestly I wouldn't even use a heating pad at all.
Dragons can't feel heat on their bellies, so it doesn't do them much good, and they are known to malfunction and overheat. Since dragons can't feel the heat on their bellies, they could be sitting on an overheating pad and burning to death. Dragons rely on overhead heat to know how hot temperatures are.
If you need to add extra heat at nighttime you will need a CHE, since the colored night bulbs are no good either.
-Brandon