In this paper we present an approach which can calculate the radius R of best-fit sphere in optical replication of off-axis aspheric surface. For regular shape, we choose the marginal points as constraint condition to get the radius R of best-fit sphere, which satisfies the root mean square of asphericity minimal. The aspheric rms under different constraints can be compared on the condition that the optical parameters, such as the paraxial radius R0 and conic constant k, are same for the parent aspheric surface of the circle, sector and regular hexagon-shaped off-axis aspheric surfaces. Moreover, we used the Golden Section Method and Cyclic Coordinate Method for optimization to solve the rms, and analyze the constraint conditions for the best-fit spheres.