Keep in mind the heat from a mat doesn't shut off or pick up instantly, so you're mainly looking for a setting with a good average. If you're putting the probe into a cup of the same medium that you're germinating in (as you should), be sure to keep it watered the same as you do the seeds themselves for a proper reading. Watch a heat cycle once you've set it to see what the peak and lowest points are. I keep my thermostat set at 84 degrees, which produces a high side maximum of 86 and a low end of 82 with an average right around 84-85.
As Masher said, 80-85 is optimum but keep in mind that many superhots will have a low germination rate or will take a long time to germinate no matter what. I had 100% germination on my Brazilian Starfish after only 6-8 days this year and most of them are already producing their first true leaves. They're going like mad. By contrast, only a single, sad looking superhot has come up thus far, a couple of Frutescens, and absolutely none of my BBG7 or Fatalii plants have awakened after a couple weeks.
As you approach 90, some types get less happy. As you drop below 80, most types take considerably longer to germinate. Don't stress over it too much, though. There are people here who just chuck their tray outside as soon as it is over 70 degrees and peppers eventually show up.
