The present invention relates to a data processing method performed by a computer, for determining geometric parameters of a phantom leg bone using a cutting guide that defines a cutting plane and that is configured to abut a predetermined surface section of the phantom leg bone. The method comprises acquiring cutting guide position data describing the spatial position of the cutting guide, acquiring relative position data describing the spatial position of a first mechanical axis point relative to the predetermined surface section of the phantom leg bone, and 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 leg bone. The present invention further relates to a corresponding cutting guide and a corresponding computer program and computer.
