Distance across the Channel is from Shakespeare Beach, Dover, to Cap Gris Nez (the headland halfway between Calais and Boulogne). This distance is 18.2 nautical miles which is approximately 21 land miles. There are 2,000 yards or 1852 mtrs to a nautical mile. Most of the England/France swims start from Shakespeare Beach or from Abbotts Cliff between one hour before high water and one hour after high water, although the pilots do start at other times and places, depending on the tide, the weather conditions, and the swimmer's ability.
France/England swims (when and if these are permitted by the French authorities) usually start from Cap Gris Nez or its immediate vicinity. Personally I like to start from about 1 mile South of the point towards Boulogne. The traditional start time is about 3 to 4 hours before high water. This can also vary considerably depending on the tide, weather, swimmer and pilot.
With the use of computerised plotting for course calculations and the modern electronics on the pilot boats, start times and places can be evaluated before the swimmer enters the water, and the best choice of route made. The more the pilot knows about the tides, weather and the swimmer's ability the more accurately the swim can be predicted.