Estimation of divergence time between organism on the synonymous substituion(Ks)

Question

[Pawan Jayaswal]

I would like to know how to estimate the divergence time on the basis of synonymous substitution(Ks) value. As far as I know for this I have followed the muse clock (=Ks/2D, where Ks : Synonymous substitution and D is synonymous mutation rate/year), here my question is how to know the value of D. Is this vary from species to species or its fixed.