The existing neutral line expressed through displacement superposition under the small deformation assumption leads to a large positioning deviation of the flexspline teeth. By taking the double disk wave generator as an example, the neutral line in the wrap angle was determined using the CAM isometric line. The outside neutral line expressed using a spline function was constructed based on the continuous conditions, such as slope and curvature, at both ends, and the difference between the tension of the neutral line was equal to the elongation produced by the circumferential force. A Finite Element Method (FEM) model of the tooth ring in contact with a circular cam was developed to verify the geometric interpolation method and the neutral line expressed with radial and circumferential displacements numerically under small and large deformations, respectively. The differences between the verification examples were calculated using the large deformation results, which were considered more suitable for the actual conditions. The examples showed that both theoretical solutions and the FEM model based on small deformation had more significant differences with the more reasonable FEM results for the large deformation case. In the 30° wrap angle model, the pole angle of the proposed method decreased by approximately 0.006°, and the pole diameter decreased by approximately 0.013 mm. The method derived based on the geometric condition in this study was closer to actual conditions and easy to implement when the model envelope angle ranged between 25° and 65°. The proposed model eliminates the influence of large geometric deformation, contact nonlinearity, and neutral line elongation in providing more accurate gear tooth positioning for the tooth profile design of harmonic drives.