Accuracy management activities are significant problems in ship hull production. A block positioning operation in a erection stage is especially the key process in hull accuracy management. The qualities of block positioning determine not only final hull accuracy qualities, but also productivities and costs in erection stage. The basic concept of computer aided accuracy management has already proposed by authors. Accuracy management activities are consisted of 'accuracy planning activities' and 'accuracy measure activities'. Accuracy measure activities are checking and coping with inaccuracy. Authors proposed the accuracy measure metrics : one is accuracy for completed (e.g. straightness, flatness) and the other is accuracy for construction (e.g. relative joint shapes of each part). In this paper, detailed accuracy metrics for a block positioning operation in a erection stage are discussed. The quality of block positioning operation is estimated by this metrics : accuracy for completed can be evaluated by the concept of the tolerance, and accuracy for construction can be evaluated by the concept of labor cost. The prototype system of accuracy metrics system is implemented. This metrics system is combined with optimization software program iSIGHT to decide the best block positioning. Then some examples of block position optimizing process are shown.