Self-mixing interferometry (SMI) is a reliable method that has been applied to measuring displacements, absolute distances, and velocities of remote targets. Evaluating the optical feedback factor C and the linewidth enhancement factor α is a vital step in calculating laser diode parameters and in processing SMI signals using phase unwrapping. This paper proposes an evaluation method for the optical feedback factor and the linewidth enhancement factor of arbitrary waveforms by investigating the slopes of phase discontinuity distribution in the optical feedback regime of 1<C. First of all, the effects that the slope of phase discontinuity distribution has on the prediction of the optical feedback factor and the linewidth enhancement factor are clarified. Next, an algorithm is proposed to evaluate the optical feedback factor and the linewidth enhancement factor using the slope variation of phase discontinuity distribution, along with a method to select discontinuities in order to improve measurement accuracy using the cumulative effect of discontinuity distribution. The proposed method is verified through simulations as well as experiments with a low-cost semiconductor laser.