The “Distance to the line” is the shortest distance from the boat to the line or its extension: it is the perpendicular to the line that passes through the bow of the boat. It is negative if the bow of the boat is beyond the line.
If the bow offset, the distance between the bow and the GPS of the phone, has not been entered, it is important to set the committee boat and the pin end buoy at an angle close to the upwind one. In such a case, in the below diagram, the blue distance and the pink one are the same.
If you have entered the bow offset, the calculation will take this distance and the heading of the boat into account when calculating the distance to the line. You are free to set the race committee boat or the pin end buoy positions in any position you want as long as the bow is on the starting line.
Today, at sea, the precision of the GPS is of the order of 5 meters. The error will not vary dramatically between the time you set the race committee boat and the pin end buoy position and the start so that the error on the distance to the line will be less than 5 meters. Starting in 2018, some of the new phones should be equipped with more precise GPS with a 30cm accuracy…