<p>It is very important to calculate the starting surface shape before machining off-axis free-form surfaces. The three-point method is usually used to calculate the starting surface shape before machining. However, the material removal of the starting surface shape calculated by this method is large. Therefore, this paper proposes a calculation method that utilizes off-axis aspherical surfaces as the starting surface profile, which is named as best-fit off-axis aspheric method (BOAS). Off-axis Zernike polynomial surfaces made of glass are analyzed. Different types of workpieces are solved separately to obtain the starting surface type with the smallest possible removal. The methodological approach proposed in this paper is validated and analyzed by means of examples. The results show that: for the non-equal-thickness type of workpiece, the maximum asphericity of the BOAS can be less than 50 microns. Afterwards, the PV can be quickly reduced to less than 2 microns by combining with the CMM inspection technique for subsequent inspection using CGH. The results show that the use of BOAS as the starting surface of off-axis Zernike polynomial surfaces can effectively reduce the amount of material removal. An indispensable method of calculating the starting face pattern for machining methods with small material removal.</p>