Just to quickly answer the original question, the radius of the 60 minute circle drawn around the origin, destination, and alternate/diversion airports for flight planning purposes is the distance the airplane flies in still air at an engine-out engine cruise speed selected by the operator and approved by their regulatory authority. The "sixty minute rule" is 14 CFR 121.161. FAA Advisory Circular 120.42B has an excellent discussion of the history and concepts behind this rule and ETOPS.