I keep my Pyxies set at 84. It warms up to ninety sometimes and as long as they have a gradient area in thier cage where it is cooler they are just fine. 85 is a good ambient temp. make sure they have clean water, clean deep substrate and a temp gradient and your frog will be fine (barring some health issue, I'm just talking general care). I've watched my Pyxies and Ornate sit in the basking spot where the surface temp is over 100. They have a big cage with a good gradient. If they want to bask in the heat they do. If they want to burrow into the low 70s temp range they do. I leave them alone and let them regulate thier bodies.
There is not a perfect temp. If your frog moves to an area of higher temp then he wants to be warmer. If he moves to an area with lower than an 85 ambient he wants to be cooler. Gradient is the key, not a "perfect temp of 79.9987898'....lol