A method is described for setting a null position of a scanning probe (4;40) mounted to the rotatable spindle (2) of a machine tool. This method may be performed as part of a probe qualification process. The method comprises setting the null position using probe measurement data collected by the scanning probe (4;40) when mounted to the spindle (2). In one embodiment, a stylus tip (14;94) of the scanning probe may be located in a conical recess (92) whilst the probe measurement data is collected. The set null position is arranged to be away from the rest position of the scanning probe and to substantially coincide with the axis of rotation (R, S) of the spindle. The need to measure and use a probe offset value in subsequent measurement cycles can thus be avoided.