A device and a method for establishing a position and orientation for a preliminary saw cut in a distal end of a femur to prepare the femur for installation of a prosthetic knee joint. A guide block defining a cutting plane is carried on an adjustably positioned locating arm that is supported on an alignment body that can be mounted on a femur either with or without an intramedullary rod in place in the distal end of the femur. Adjustment screws and horizontal slots in the alignment body provide for varus/valgus angular adjustment of the position of the alignment body. An adjustment guide block may have a saw guiding face defining a differently oriented cutting plane.