One of the major drawbacks of adding signal techniques of reducing peak to average power ratio (PAPR) is the variation of the average power of the transmit signal that undoubtedly degrades the performance of transmission. In this paper, assuming that the baseband OFDM signal is characterized as a complex Gaussian process, we derive an approximation by a simple closed form of the variation of the average power for geometric method [1] of PAPR reduction known as an adding signal technique. Comparisons of the proposed approximation with computer simulations show good agreement and convergence with an increase in the magnitude threshold.