A computer aided plate bending system is under developing in authors' company to realize the automatic forming of shell plates in shipyards. The Finite Element Method based on the theory of inherent strain has been proved to be a very powerful approach to obtain proper inherent strains which govern deformation. In practice, the inherent strains are approximately achieved through combination of many single heating lines, leaving certain geometric error. In this paper, geometric errors produced in two processes, and the ways how to reduce them are discussed. In the stage of planning heating lines, converting inherent strains into heating lines causes some error in deformation. It is because that the inherent strains calculated from plate expansion by FEM could be arbitrary while the shape and magnitude of the inherent strains produced by heating lines are of limited types. Iterating calculation is used to eliminate this error. Another kind of error comes from heating stage. The plastic strains produced in real plate are commonly different from those obtained from experiments even though heat input and heating time are same. Some factors, such as residual stress, initial curvature and effect of heating position have fairly large influences. They are usually too complicated to be predicted in planning stage. Instead of separate investigations, a statistical method is proposed to evaluate whole system according to the deformation resulting from previous line-heating. Period-by-period prediction and refining arrangement are therefore possible. A saddle shaping experiment shows that the geometric errors in both the stages are very large. And the above mentioned methods are proved to be effective in decreasing the errors.