Just guessing out loud here, but if you go to airnav.com and look up any airfield, two points are given for opposite ends of the runway. For MOR or Morristown, TN, Runway 5 = Latitude: 	36-10.456562N, 	Longitude: 	083-22.968408W, Elevation: 	1312.9 ft. and the other end, Runway 23 = Latitude: 36-11.069353N, Longitude: 083-22.085700W, Elevation: 	1274.4 ft. As an Engineering Consultant, everything that is done on this runway is done considering these two points - measurements for placing markings, setting up the RSA and all Part 77 surfaces and safety lines, etc. Approach surfaces, departure surfaces, everything is established off these two points, including length of runway, TODA, TORA ASDA, LDA, etc. are all configured off these FAA established runway endpoints, whether there is any extra pavement beyond or not. In my estimation, the DER is going to be the opposite end runway coordinate of the end you are rolling out on.