Calibration should take place annually and/or any time a change is made to the equipment or the material. The City of Cudahy reduced salt use by 50% through calibrating their equipment. See their instructional video, and videos from other communities below.
Tips for calibrating a Force America 6100 or 5100ex (City of Cudahy).
Calibration for a manual drop tail gate spreader with manual valves (City of Farmington Hills Department of Public Works).