A semiempirical formula for neutron detector responses, to be used to infer reactivities in subcriticality measurements, is presented. A formal theory for the multipoint approximation of the Boltzmann operators makes possible the description of a large variety of nuclear fuel systems by means of an equivalent two-point model that regards a whole system as the coupled system made up of an arbitrary number of nuclear fuels. Because the analytic formula includes the fitting parameter associated with the detector configuration and because the removal of spatial effects or higher mode contaminations in the detector responses is accomplished by devising the detector configurations, the conventional point approximation can be used to infer the reactivity of a far-subcritical system. For an example of an application to existing experiments, the current method is used to analyze subcriticality measurements by using the 252Cf source-driven neutron noise analysis method.