Disclosed is a method for determining location of a lens machining tool (24) having an offset location according to a first direction (Y) smaller than a first predetermined threshold, including the steps of manufacturing a calibration piece (10) according to a predetermined theoretical geometry by using the lens machining tool for providing a at least partially annular groove in a main surface of the calibration piece, the at least partially annular groove being configured to form at least one sharp edge defining a slope discontinuity on the main surface; measuring a distance between the at least one sharp edge and a turning center of the calibration piece for providing data of geometrical characteristics of the calibration piece; and deducing from the measured data a location of the lens machining tool according to a second direction (X) distinct from the first direction.