In this paper we present a performance dimensioning guide for a large telecommunication application - a billing gateway. We compare two approaches: a parametric approach where the performance predictions are based on linear combinations of the performance implications of certain parameters (i.e. the amount of filtering and formatting in the configuration), and a holistic approach where the predictions are based on recordings from a number of representative configurations. The predictions are used to select the appropriate hardware platform. We compare the two approaches for three configurations. These evaluations show that the holistic approach is more accurate and easier to use.