hiya mate.
You want to use a heat mat and a thermostat for cresties to get the temps.
basically the temp on the surface of the mat will be higher but forgetting that for a moment you want to aim for air temps ranging from 70-85 during the day, must have that cool bit which is why i think ppl are mentioning not needing heat.
they will be ok at 75 ish all day long but not at 85 u see.
65-70 al over at night.
if you use a heatmat that only takes up a 3rd of the floor of the viv, with an exo terra bein glass the heat will go out quick which is ideal in this case.
have the sensor of the stat in the warm end half way up the viv and set it to 80-84 see what worls best by using a digital thermometer to check temps in various places at various times over the course of a few days and u should get it spot on.