Generation reserves are the key resource for balancing power system generation and consumption. Reserves are used to mitigate any imbalances due to uncertainty and variability. However, as a costly commodity, the amount of reserves should be carefully assessed to maintain the cost of power system operations at its minimum. Currently, reserve requirement determination is based upon a posteriori methods that use the experience of power system operators and established assumptions. While these assumptions have been made out of a level of engineering practicality, they may not be formally true given the numerical evidence. The two prequels to this work presented methods for determination of the load following and ramping reserve requirements a priori. This paper now uses a similar methodology to determine the regulation reserve requirement.