Hi Robbie,
It's great to see you've started a Basho forum, many thanks for
setting it up.
I play a lot of 12 string guitar, and I'd like to share my
observations about tuning, specfically re Variations on Clair de Lune.
My approach to 12 string is actually a reasonably common one. I use
medium gauge strings ( 13 - 56 ) paired with proportionally heavier
octave strings and tune the whole guitar down 2 whole steps ( 4 frets
). This places the guitar in std tuning with but C as your open 6
string note. The guitar is now tuned ( from 6th to 1st ) C, F, B flat,
E flat, G, C. I believe Robbie Basho did this as well, as did Leo
Kottke, and Lead Belly ( who used very heavy strings and tuned to B)
From this low standard tuning, I then retune the guitar into various
open or altered tunings.
With regard to Clair de Lune, I take my low tuned 12 string and use
the following approach. 6th string lowered 2 whole tones, 5th lowered
one whole tone, 4th lowered 1 whole tone, 3rd as is, 2nd raised 1
semitone, 1st as is. This is analogous to open C when in E std tuning.
The 12 string guitar is now tuned to ( 6th to 1st ) A flat, E flat,
A flat, E flat, A flat, C.
Try playing the 12th, 7th, and 5th fret harmonics as Robbie does as
the end of the song.
This is actually more complicated to explain than it really is, but
I hope I have been reasonably clear.

All the Best for now
Andy Miller