The paper deals with the study of the optimisation of the distribution of the download time from a particular Video on Demand System. This VOD system is based on Grid Delivery Network which is an hybrid architecture based on P2P and Grid Computing concepts. In this system, the data are shrunk into fixed size blocks which must be replicated on hosts to decrease the total download time. We propose an optimal replication factor to optimise the average download time. We showed that the allocation methods are directly correlated with the variance of response waiting time. We analyse different heuristics to solve these two problems in practice, and validate them through simulation. With our methods, we can guarantee a pre-established waiting response time with an error of approximatively $10 \%$.