Abstract The critical distance value should theoretically be determined from the plain specimen fatigue limit and the threshold stress intensity factor, though usually ordinary notch geometries are considered. In this paper, we proposed an optimized sharp notch with the aims of simple and reliable manufacture and, more importantly, a local strong stress gradient able to minimize the sensitivity on the deduced critical distance value. A numerical procedure is proposed to find the critical distance from the fatigue strength of the notched specimen, by implementing the line method with simple formulas based on dimensionless equations and specific coefficients derived from accurate FE analyses. A definition of the boundaries for a valid critical distance evaluation is also introduced and discussed. Finally, an application example is provided on a quenched and tempered steel also comparing the obtained critical distances with the threshold derived values.