The Micro-Electronics Facility Efficiency Workshop (1995) covered the topic of an optimum operating point for chillers.
... high efficiency [Cooling] towers [can be used] with variable speed drives ganged together and controlled to take advantage of favorable ambient air conditions to lower the condenser water temperature. However the cost-effectiveness of this strategy depends greatly on the specific climate and in particular the wet-bulb temperature. Since cooling towers get more efficient with higher leaving condenser water temperatures while chillers get less efficient, …the concept of an optimum operating point that maximizes the combined performance of the chiller and cooling tower [must be implemented]. With more efficient cooling towers, this optimum point can be dropped to a lower condenser water temperature which will further improve the overall system efficiency.