For a whites treefrog I’d honestly prefer a basking lamp on one side of the enclosure over a heat mat. You’re aiming for an ambient temperature in the high seventies during the day with a warm spot near eighty on one end. It’s easier to maintain this with an appropriately sized bulb in my opinion. Since they are a larger species and require an enclosure near 20 gallons for 2 adults a heat mat isn’t always the best option. It’s just not very good at heating the air.

I do use a 8 by 4 heat Matt on the side of my juvenile red eyed tree frog enclosure though. Since that enclosure is smaller (12x12x18) the heat mat works since a bulb would be overkill. I aim for a temperature around 84 on the glass which translates to near 80 a few inches away.

Get a a good temp gun and thermostat. It may take some work but you’ll be able to fine tune it.