A data processing method performed by a computer, for determining geometric parameters of a phantom tibia or a phantom femur of a phantom patient, using a cutting guide that defines a cutting plane, that is configured to abut a predetermined surface section of the phantom tibia or the phantom femur and that comprises a marking that is adapted to be brought into alignment with an anterior-posterior direction of the phantom patient, the method comprising the following steps: - acquiring cutting guide position data describing the spatial position of the cutting guide abutting the predetermined surface section, and with its marking being aligned with the anterior-posterior direction, the predetermined surface section of the phantom tibia being represented by the anterior tibia cortex, or the predetermined surface section of the phantom femur being represented by the most distal condyle point of the phantom femur the method being characterized in that it further comprises the steps of: - acquiring, from an anatomical atlas describing a three-dimensional general model of the phantom tibia or the phantom femur, or from a two-dimensional image of the phantom patient taken in a predefined direction, relative position data describing a distance between a first mechanical axis point and the predetermined surface section, with the distance being a distance from the anterior tibia cortex in the anterior-posterior direction, or with the distance being a distance from the most distal condyle point of the phantom femur in a direction parallel to the anatomical axis of the phantom femur, wherein the three-dimensional general model or the two-dimensional image is scaled to the actual phantom tibia or the phantom femur based on a determined length of the phantom tibia or the phantom femur; - determining, based on the cutting guide position data and the relative position data, first axis point position data describing the spatial position of the first mechanical axis point of the phantom tibia or the phantom femur. - acquiring cutting guide position data describing the spatial position of the cutting guide abutting the predetermined surface section, and with its marking being aligned with the anterior-posterior direction, the predetermined surface section of the phantom tibia being represented by the anterior tibia cortex, or the predetermined surface section of the phantom femur being represented by the most distal condyle point of the phantom femur the method being characterized in that it further comprises the steps of: - acquiring, from an anatomical atlas describing a three-dimensional general model of the phantom tibia or the phantom femur, or from a two-dimensional image of the phantom patient taken in a predefined direction, relative position data describing a distance between a first mechanical axis point and the predetermined surface section, with the distance being a distance from the anterior tibia cortex in the anterior-posterior direction, or with the distance being a distance from the most distal condyle point of the phantom femur in a direction parallel to the anatomical axis of the phantom femur, wherein the three-dimensional general model or the two-dimensional image is scaled to the actual phantom tibia or the phantom femur based on a determined length of the phantom tibia or the phantom femur; - determining, based on the cutting guide position data and the relative position data, first axis point position data describing the spatial position of the first mechanical axis point of the phantom tibia or the phantom femur.