The values of bond valence parameters R-0 for the bonds of main group metal halides are usually obtained by systematic analysis of the crystal structure data by assuming B=0.037 nm. In this work, conversely, the values of B were proposed to be fitted with crystal structure data on assuming the values of R-0 equal to the bond lengths of r(g) accurately determined by gaseous electron diffraction. As an example, the new sets of R-0/B for the Pb(II)-X (X=F, Cl, Br, I) bonds were obtained and fitted as 0.2036/0.0382, 0.2447/0.040, 0.2598/0.040, and 0.2804/0.0386, respectively. Compared with the values of R-0=0.203, 0.253, 0.264, and 0.278 nm based on B=0.037nm for Pb(II)-X bonds reported in the literature, these new parameters were not only accurate and reliable for bond valence calculations, but also revealed the physical meaning inherent in every R-0 as unit valence bond length
