In optical design, the purpose of tolerance allocation is not only to satisfy all image quality requirements but also to minimize the machining cost. Therefore, to achieve these two goals easily and quickly, in this paper, we propose a calculation method that can effectively reduce the data storage amount and relatively accurately fit the function relationship between tolerance and image quality. At the same time, distortion is used as the performance criterion. The differential derivation formula of the distortion with respect to the two structural parameters of decenter and tilt is derived to improve and supplement the types of structural parameters. An example is used to demonstrate the feasibility and flexibility of the approach, which can achieve a balance between image quality and machining cost.