g-tensor components for a Kramers doublet of an impurity ion doped into dielectric crystal are expressed through coefficients of expansion of the doublet wavefunctions in the basis of full momentum of the impurity ion taking into account isotropic reduction of orbital momentum in Zeeman energy due to covalence. Mixing of terms and multiplets of the impurity ion is considered rigorously in the expansion of doublet wavefunctions. The obtained expressions can be useful in calculations and analysis of g-factors dependence on the impurity ion Hamiltonian parameters. The derived formulas are applied to calculation of g-factors of the ground Kramers doublet of Ce3+ ion doped in LiYF4 crystal. It is shown that considering reduction of orbital momentum of the 4f electron of the Ce3+ ion in Zeeman energy can significantly improve agreement with experimentally measured g-factors for this compound, available in literature.