Keep in mind also that when one plays dobro the hand that holds the bottle neck (or whatever) damps the strings on the nut side of the hand. The guitar is set up so that at fret 12 the string is 'cut in half', thus halving the string and so doubling the rate of vibration, hence providing an octave. So, in short, any place you halve the string will create the octave, so the placement of the nut in relation to the 12th fret (or any of the others) is not crucial. It then becomes relativistic as to where one finds the various chords (notes) on the string.