We describe a modified six-step method to simultaneously measure the inhomogeneity of sample plate and the planarity of the four surfaces in an absolute manner, along with a high-efficiency iterative algorithm for data reduction. Combined with the iterative algorithm, the errors of inhomogeneity and flatness can be estimated with pixel-level spatial resolution in a fast and effective manner. The simulation and experiments prove the validity of the method and the measurement capability of reaching sub-nanometer accuracy. The method presented in this paper is cross-compared with traditional absolute testing method and the method of inhomogeneity. The difference between absolute plane measurements is less than 1.7 nm RMS, and the difference of inhomogeneity measurement accuracy is less than 2.3 nm RMS. The experimental results show that these two methods are highly consistent and have good repeatability, which verifies the accuracy of the methods proposed in this paper. Uncertainty analysis indicates that the proposed method improves the measurement uncertainty, compared with the classical transmission method.