Beats me, I'm afraid !! I just stick it in double-dropped-D tuning,and experiment with 2 & 3 finger chords until I find something I like the sound of ! Dunno what they"re called, though. I can pick out the notes for most English,Scottish and Irish dance tunes in that tuning, and use a capo a lot to find other keys ( 2nd fret, G shapes for A, D shapes for E, 5th fret D shapes for G and so on)----I do "Lowlands" on the 7th fret !!! ( with another,conventionally-tuned guitarist, of course !! ).Sounds good, too !
                         Mr D ( as Derek Tarrant calls me !).