In the 2005 paper by Mayes et al., a threshold-based performance control system (PerCo) was described and an initial experimental evaluation was presented. The objective of the current paper is to investigate the role of the threshold value in PerCo and to place the threshold-based rescheduling heuristic on a more principled footing. Simulation enables us to identify the 'optimal' threshold value for a particular application scenario, and we show that this optimal value results in a 10 per cent improvement in performance for the application considered by Mayes et al. Furthermore, we find that the execution time of this optimal threshold-based schedule is very close (within 0.5%) to the execution time that results from a linear programming optimal schedule.