It depends a lot on how warm or cold it is in the particular room that they're placed in etc. There's no standard answer for this. The best advice I can give you is to put all of your heating devices on a thermostat and then they will turn on and off as needed to keep it the appropriate temperature.
it shouldn't be confusing at all. You probably don't need an overhead heat source at all (unless it gets really cold in your house). Start by getting a good indoor/outdoor digital thermometer so you can tell specifically what the current temperature is without any heating device. Then add an appropriately sized under tank heater (uth) with a thermostat. You set the thermostat to the desired temperature and it will turn the heat pad on and off as needed. If you're still not producing enough heat to get the temperature up to the appropriate levels, then you might consider some sort of over head heating. Here's a good example of some of the available products on the market: reptile heating products, thermostats & thermometers