This paper presents a probabilistic model to assess the co-channel interference produced by multiple FWA transmitters into a HAPS victim receiver. To avoid the worst assumptions usually considered in traditional interference calculation methods, in the proposed approach the geographic positions of FWA base and subscriber stations are characterized by two dimensional point processes. Two specific interfering scenarios are considered and, in each of them, coordination distances obtained with the proposed model are compared to those resulting from the traditional deterministic interference calculation methods.