The statistical 1NN method is an elegant way to derive the composition of small B-enriched clusters in a random AB solid solution from 3D atomic fields. An extension of this method is proposed that includes the contribution of interface region and provides an estimate of the core composition of clusters. This model is applied to boron-implanted silicon containing boron-enriched clusters. A comparison with the previous model is performed. This new approach gives relevant information, i.e. the core composition of clusters and the cluster-matrix interface width.