Flexwatt can get extremely hot if left at full power, so placing the probe directly on the heat tape will keep inside temps from getting too hot. You may have to set it higher than desired temp because the heat has to transfer through plastic and substrate. I measure temps at plastic itself in case the snake decides to burrow under bedding or newspaper. Measure at the hottest place your snake can find itself.
Spending the extra money on a Helix or Herpstat is a good investment IMO.