  The NPDGamma experiment measures the parity-violating asymmetry in
$\gamma$-ray emission in the capture of polarized neutrons on liquid
parahydrogen. The sensitivity to the asymmetry for each detector in the array
is used as a parameter in the extraction of the physics asymmetry from the
measured data. The detector array is approximately cylindrically symmetric
around the target and a step-wise sinusoidal function has been used for the
sensitivity in the previous iteration of the NPDGamma experiment, but
deviations from cylindrical symmetry necessitate the use of a Monte Carlo model
to determine corrections to the geometrical factors. For the calculations,
source code modifications to MCNPX were done in order to calculate the
sensitivity of each cesium iodide detector to the physics asymmetry. We
describe the MCNPX model and results from calculations and how the results are
validated through measurement of the parity violating asymmetry of
$\gamma$-rays from neutron capture on chlorine.
