As stated, there is not much point in trying to calibrate if you don't have an accurate thermometer. I invested years ago in a very accurate digital one from Homershams (accurate to 0.1 deg C)
If they are the old jeager type with a button on the top you place them in water of a known temperature until the whole thing is the same temperature as the water of known temperature. You then pull the button up and turn the dial to match the known temperature. Push the button down and it is calibrated.