How precise do you want to go? If you want simple, stick the thermo in boiling water and adjust the screw to 212.
If you have a thermo for a chiller, you may want to calibrate to 32 because 70 is closer to freezing than boiling. Stick it in ice water and adjust the screw.
The next step for precision is to find your altitude and then use a calculator like this to find your boiling temp. You can get your altitude from your city's Wiki page and possibly your specific house from google maps.
I have all of my thermos calibrated to boiling at a hair below 211 because I'm about 700 ft above sea level.
If you want super precision, you'll need your altitude, a good barometer reading, some graph paper, and both boiling water and ice water.