We propose a mathematic formula that can be applied to work out the amplitude distribution of a tested laser beam from its radial-shearing interferogram accurately, even if the amplitude distribution is extremely uneven. Provided that the background irradiance distribution of a tested beam is extracted from its cyclic radial-shearing (CRS) interferogram first, then using the radial-shearing ratio of the CRS interferometer, the amount of lateral shear along the orthogonal directions and the irradiance ratio, the amplitude distribution of a tested beam can be reconstructed accurately by the proposed formula. The simulation computation and experimental results show that the formula allows any amount of lateral shear to happen, as long as the contracted beam maintains its location inside its corresponding expanded version.