You will have to pop the mat in, with a thermometer- prefereably digital, turn the stat right up or right down, then adjust it every hour or so (whenever the temp stops fluctuating) until you get the temperature you're after.
The probe should be right on the heatmat/substrate as that's where the leo will be, same with the thermometer.
Trial and error unfortunately... even the ones with temp dials on aren't accurate. Mine was miles out... took me 3 days of adjusting but eventully sorted it out.

Good luck!