We have computed a grid of synthetic spectra in the wavelength range {lambda}{lambda}4600-5600{AA} using revised model atmospheres, for a range of atmospheric parameters and values of [alpha-elements/Fe]=0.0 and +0.4. The Lick indices Fe5270, Fe5335, Mgb and Mg_2_ are measured on the grid spectra, for FWHM=2 to 8.3{AA}. Relations between the indices Fe5270, Fe5335 and Mg_2_ and stellar parameters effective temperature T_eff_, logg, [Fe/H] and [alpha/Fe], valid in the range 4000K>=T_eff_>=7000K, are presented. These fitting functions are given for FWHM=3.5 and 8.3{AA}. The indices were also measured for a list of 97 reference stars with well-known stellar parameters observed at ESO and OHP, and these are compared to the computed indices. Finally, a comparison of the indices measured on the observed spectra and those derived from the fitting functions based on synthetic spectra is presented.