Leptonic decay widths for pseudoscalar mesons are calculated in a QCD-motivated Bethe-Salpeter formalism on a two-tier basis. This model, which is characterised by a Lorentz-invariant confining kernel that depends on the relativistic reduced mass defined in accordance with the Wightman-Garding definition of the internal 4-momenta q_mu, has already shown precision fits to the spectra of qq, qQ and qqq hadrons apart from exhibiting an explicit Regge-asymptotic behaviour (M² approximately N). The leptonic decay constants thus obtained with no free parameters, are (in MeV units), f_P=134( pi ), 166(K); 158(D); 94.9(B), 114(B_s). The decreasing trend with increasing mismatch in the constituent quark masses seems to be in accord with the latest lattice results.