We investigate the momentum transfer dependence of differential cross sections dσ/d t in diffractive electroproduction of heavy quarkonia on proton targets. Model predictions for dσ/d t within the light-front QCD dipole formalism are based on a realistic model for a proper correlation between the impact parameter b of a collision and color dipole orientation r. We demonstrate a significance of b − r correlation by comparing with a standard simplification b r, frequently used in the literature.