Results of some 3500 numerical solutions of Tc for 64 different effective phonon spectrum a2F(ω) are presented. It can be seen from these results that the Tc has the conditional maximum (Tc/A)max, if the area of spectrum is unchanged. Tc depends chiefly on the position and area of the peak of spectrum. Its dependence on the peak width is comparatively unimportant. If the double δ spectrum is replaced by the double ω spectrum with same and 2>, the rms error is 3.2%. In this paper I analysis and settle the dispute regarding the "λ=2 limit" in the literature.