A major problem in mobile computing is the optimal distribution and replication of data files in the Grid sites, in order to improve and maintain over time a high overall throughput of Grid jobs that access files. Therefore, a Grid optimization service should be able to dynamically modify the geographic distribution of data files, triggering file deletion and replication, according to the variation over time of the sites where data is highly requested. In this document we propose a novel prediction function for evaluating the future usefulness (value) of data files. This function can be used by Grid sites to make informed decisions about whether or not to replicate files locally. We have performed some preliminary tests on the accuracy of this function using randomly generated simulated file access patterns and also compared the predicted values with the simulated values. It turns out that, over the performed tests, the novel function predicts the simulated values reasonably well and has big advantage than former functions.