Cluster tools are representatives of a special kind of tool where process times of jobs depend on the combination in which they are processed together on the tool and hence, depending on the sequence in which they are processed at a tool. To evaluate schedules of jobs to be processed at such a tool an estimation method is needed since a detailed simulation takes too long. In this paper, we present a method based on slow down factors which produces promising results and gives hints for the development of intelligent scheduling methods for this kind of tools.