Temperatures, Heat Mats, and Thermostats?

[mzh3de]

So, probably a stupid question, but better safe than sorry.

I should be getting a new BP in about two weeks, I'm getting an Ultratherm Heat Mat, and I'm new to thermostats. What temperature (celsius) should I set it (the thermostat) to?

Current ambience of the tank seems to be staying at around 20-25 degrees, I'll update if that changes at all.

Oh, and if anyone has any tips for checking thermometer accuracy, that would be appreciated too

Thanks!