OK, for those who keep up-to-date with the main site, I published "Making a Glass Capo for a Fretless Guitar" and explained how it could only work if you replaced the original strings with a new set specifically designed to be flat to the glass capo.
I've since restrung the guitar and replaced the old nut with a bone nut. These smell really bad when you cut them with a Dremel and it took me a couple of hours to sand the nut and cut the slots.
However it works really well and you can read about the string gauges here:
I've tuned the guitar to Open G7 tuning and this has been added to "Alternate Tunings for Fretless Guitar"