Thermometer is not only the control panel that you use to change the temperature, but it is the central hub of the whole HVAC system. With the climate that I experience in Las Vegas, for instance, temperature fluctuates between 30°F and any other number within a single day, hence the need to calibrate the device accurately. 

Signs of Thermostat Issues 

• Short cycling: This can be a calibration problem if, for instance, your AC is switching on and off frequently. 
• Temperature Discrepancies: You can follow this simple rule: every half an hour, check the temperature your portable thermometer shows to the temperature that is set by your thermostat. 
• Uneven Cooling: For instance, should some parts of the house be cooler than others, then thermal might be depicting the real temperatures of the house wrongly. 

Calibration Tips 

• Location Matters: Check also that your thermostat stays clear of direct sunlight or heat sources like a stove or oven. This results in false readings that mislead users and are therefore dangerous. 
• Level Check: On the specific case of hanging the thermostat, if you are not sure of the level surface, try to use bubble level. It is extremely sensitive, and any tilt, even a very small one, can distort it. 
• Professional Calibration: In order to get the best results, which is especially true for the older thermostats, it is advised to use the professional service of calibration that may include the usage of the special equipment.