Optical lattice atomic clocks have achieved remarkable fractional frequency uncertainties, playing a vital role in precision metrology and fundamental physics. In the closed-loop operation of an optical clock, stabilizing the clock laser to the ultra-narrow atomic transition is essentially a parameter estimation problem. Although measurements are conventionally performed at the full-width at half-maximum (FWHM) of the Rabi spectrum to maximize the discriminator slope, it remains unclear whether this point provides the maximum Fisher information under the influence of system decoherence. In this paper, we theoretically and numerically investigate the Fisher information of the Rabi spectrum under varying decoherence strengths. To accurately quantify the decoherence effect, we introduce a coherence contrast parameter α (\alpha \in (0, 1). By analyzing the physical competition between the spectral line slope (representing signal sensitivity) and the quantum projection noise (QPN), we reveal the underlying mechanism governing the optimal measurement point. We find that under ideal fully coherent conditions (\alpha = 1), the maximum Fisher information is located exactly at the resonance center. However, when decoherence is present (\alpha \lt 1), the non-zero QPN at resonance forces the maximum Fisher information point to shift towards the FWHM to seek a larger spectral slope.

To evaluate the practical enhancement of this mechanism, we further construct a numerical simulation of the clock laser locking process. The simulated Allan variance confirms that tracking the dynamically optimized detuning point (e.g., \Delta\delta = 0.653g at \alpha = 0.9) effectively reduces the Allan variance compared with the traditional FWHM measurement (\Delta\delta \approx 0.8g). Our study proposes a dynamic optimization strategy that significantly improves the stability of optical lattice clocks purely through measurement scheme optimization, without requiring any additional hardware modifications.