This technical note aims at optimizing the concentration ratio of multi-faceted focusing heliostats implemented into a solar tower power plant. The ideal shape of a heliostat located off-axis in the field is known to be the local section of a fictitious paraboloïd whose parameters are varying continuously with the Sun angular position. We describe an optimization procedure applicable to those heliostats. The flux densities formed at the solar receiver and the achievable concentrating ratios are computed using an improved convolution algorithm. It is shown that the optimized heliostat shape can produce typical concentration gains of approximately 10%, even when the heliostats reflect the Sun under large incidence angles.