The values used for the radius of the Earth (3961 miles & 6373 km) are optimized for locations around 39 degrees from the equator (roughly the Latitude of Washington, DC, USA). Use LatLong.net to find the Latitude and Longitude for any U.S. address and DistanceFrom to find as-the-crow-flies distances. Also, I wrote a script to convert between decimal degrees and degrees/minutes/seconds formats. First location (default: 1600 Pennsylvania Ave NW, Washington, DC)