FYI: Presuming that the string height is not overly high - which I doubt- on your Martin; I would consider a "Hamilton Fret Spanner Capo". This capo clamps directly on top of the fret wire and tightens with a screw device at the back. This eliminates excessive string stretching and although slow to put on I have found much more suitable. Have used it extensively with electric guitars with very acceptable results.