Several principal conclusions for the so-called core-wing-ratio method for measuring the roughness of diffusing surfaces with laser beam based upon the previous papers are given. The method for determining the coefficients in the probability density function P(h) of random height is given, following the emperical relation R2=5Ra. For P1(h)={e(-b(|h|/hm)) ,|h|≤hm 0,|h|>hm and P2(h)=e(-a2(h/hm)2) is determined that b=1.23 and a2=2. It is shown that in both P1(h) and P2(h), hm is corresponding to 1/2Rz. The core-wing-ratio values D2/D1 as functions of Rz are deduced and computed for P1(h) and P2(h) with formulas which are more precise than that in the previous papers. In addition, the relation of the half width of the core of diffraction pattern with Rz is computed, and it is pointed out that the half-width method is not convenient for surface roughness measurement.