(modified 8/2014)
...To appoximate the proper tail gut length, adjust it so that the distance from the center of the bridge to the string bar on the tailpiece is 1/6 th the length of the string from the nut to the center of the bridge. Since this 'stopped length' on a full sized violin is (12 15/16"), 2.156" is the other measurement (2 5/32") from the bridge center to the string bar of the tail piece.
...Remember that the new tailgut will stretch with string tension so make the tail gut a bit short to begin with. As a practical guide, adjust the tail gut length as short as possible so that the tailpiece just does not touch the saddle. Then under full string tension is will have just a little clearance to the saddle.