Devising a model to figure out metaheuristics performances when applied to high demanding problems is paramount. Such a model would allow to foresee the computational time required for a given output quality. In previous works we showed that the performance of CHC and PBIL evolutive algorithms fit in a Gamma distribution when applied to solve the Root Identification Problem for search domains bounded by 220 instances. Here we conduct experimental studies to verify that the Gamma model applies to search spaces of up to 250. Then we apply the model to estimate the runtime for search spaces of size up to 2100. Finally we run the algorithms and validate the model by comparing the actual runtime values to those yielded by the model.Postprint (published version