Robert, the 'scale length' is the length of the vibrating portion of the open string, measured from the saddle to the nut.
This is true, however it is not the most accurate way of determining scale-length because of the differences in this length for each string, depending on where the saddles are set for proper intonation.
A better way of finding scale-length is to measure from the inside edge of the nut to the center (the crown) of the 12th fret, and then doubling that length.