Digital signal processing (DSP) is a major driving force for cost-effectively realizing “software-defined anything” required by future converged networks. The fast Fourier transform (FFT) is a fundamental building block of an overwhelming majority of those DSP algorithms. For practical real-time implementation, the logic resource usage reduction of FFT operations is critical for considerably decreasing the hardware cost and power consumption. In this paper, a simple and effective solution of stage-dependent minimum bit resolution of full parallel variable-point FFTs is analytically derived, for the first time, whose validity and robustness are rigorously verified, both numerically and experimentally, over intensity modulation and direct detection (IMDD) optical OFDM transmission systems. The developed solution has unique advantages including great simplicity, excellent accuracy and robustness, and significant saving in logic resource usage. The solution can ease the practical real-time FFT DSP design, decrease the DSP complexity and maximize the overall system performance by making full use of available transceiver/system design parameters.