A heat mat has a maximum temperature that it can reach. A thermostat can cause the heat mat to shut off to let the temperature go below that maximum temperature but a thermostat is not a heating device so it can't make the heat mat hotter.
The best way to determine the floor temp:
--invest in a temp gun
or
--get a digital thermometer with a probe. Put the probe on the floor of the tank and give it a few minutes to register correctly.