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Beyond the scope of space-coding metasurfaces, space-time digital metasurfaces can substantially expand the application scope of digital metamaterials in which simultaneous manipulation of electromagnetic waves in both space and frequency domains would be feasible. In this paper, by adopting a superposition operation of terms with unequal coefficient, Huygens principle, and a proper time-varying biasing mechanism, some useful closed-form formulas in the class of large digital metasurfaces were presented for predicting the absolute directivity of scatted beams. Moreover, in the harmonic beam steering scheme, by applying several suitable assumptions, we have derived two separate expressions for calculating the exact total radiated power at harmonic frequencies and total radiated power for scattered beams located at the end-fire direction. Despite the simplifying assumptions we have applied, we have proved that the provided formulas can still be a good and fast estimate for developing a large digital metasurface with a predetermined power intensity pattern. The effect of quantization level and metasurface dimensions on the performance of power manipulating as well as the limitation on the maximum scan angle in harmonic beam steering have been addressed. Several demonstrative examples numerically demonstrated through MATLAB software and the good agreement between simulations and theoretical predictions have been observed. By considering the introduced restrictions in the manuscript, this method can be implemented in any desired frequency just by employing phase-only meta-particles as physical coding elements. The author believes that the proposed straightforward approach discloses a new opportunity for various applications such as multiple-target radar systems and THz communication.