Accurate determinations of fusion neutron yields on the Tokamak Fusion Test Reactor (TFTR) require that the neutron detectors be absolutely calibrated in situ, using neutron sources of known strengths. For such calibrations, numerical simulations of neutron transport can be powerful tools in the design of experiments and the study of measurement results. On the TFTR, numerical calibration experiments are frequently used to complement actual detector calibrations. Calculational approaches and transport models used in these numerical simulations are presented and the results from a simulation of the calibration of 235U fission detectors carried out in December 1988 are summarized.