The 12th fret is exactly half way down the string. If you are getting the octave and harmonic at 13, then you need to move the saddle forward, towards the pups and neck. Measure the 0-12 fret distance, and then measure 12 fret to saddle. they should be the same, but it sounds like your first measurement will be short, by about 1/2". B saddle on most guitars is farther back than the others, about level with that of the 6th string. Measuring is just a rough guide - it needs doing as you were, using 12th fret harmonic. Some actually check by playing 12 fret note, then fretting on 13, and plucking the string above the fretboard. It's a good guide.