  We investigate nitrogen-vacancy center (NV) ensembles in diamond under the
influence of strongly-correlated electron-spin baths. We thoroughly calculate
the decoherence properties of the NV central spin for bath concentrations of
0.1-100 ppm using the cluster-correlation expansion (CCE) method. We
systematically analyze possible origins of the significant deviations in the
values of the $T_2$ coherence time reported in literature. We demonstrate that
significant variations can originate from the choice of averaging and fitting
procedures used for the ensemble average and we point out the respective
aspects that need to be considered, when comparing the various theoretical
studies. Our study may ease readers to perform reliable and fast simulations on
the central spin problem. It provides an understanding and interpretation of
the outcome parameters describing the dynamics of the local bath spins.
