Helical and helicoidal precipitation patterns emerging in the wake of reaction-diffusion fronts are studied. In our experiments, these chiral structures arise with well-defined probabilities P(H) controlled by conditions such as, e.g., the initial concentration of the reagents. We develop a model which describes the observed experimental trends. The results suggest that P(H) is determined by a delicate interplay among the time and length scales related to the front and to the unstable precipitation modes and, furthermore, that the noise amplitude also plays a quantifiable role.