I recently gave my first nylon crossover performance, and 41.5mm string spacing at the nut is definitely my favorite. Playing was free and easy. So, I definitely recommend a nut width of 48.5mm.

One interesting point is that a nut width of 48.5mm and a string spacing at the saddle of 57mm is the standard for Emerald's X20 Nylon, and these are precisely the dimensions I would recommend for a production guitar for fingerstyle players. As is typical, their web site does not publish neck width at the 12th fret. Personally, however, I wouldn't recommend a crossover that is as big as, or has a scale as long as an X20 Nylon. A 12 fret, 640mm scale length guitar with a lower bout less than 15" is in my comfort zone because it allows my left arm to hang loosely by my side, and doesn't require my right arm to wrap around a big box.

On the topic of removable necks, nylon strings respond very slowly to changes in tension. While my guitar does not have a removable neck, I have been continuously loosening and tightening strings during the making and swapping out of nuts and saddles. It took hours for pitch to stabilize. I also broke some wound strings, not because of sharpness at the nut or saddle (they didn't break at the nut or saddle). They just couldn't take the drastic changes in tension over and over again.

Conversely, I regularly perform along with ethnic instruments that are not in standard pitch, and retuning electric strings produces stable results in minutes.

Tom2;

I have owned three Emerald nylon string guitars and have found, as you note, that the nut width and string spacing are as good as it gets.

I have gone with the longer scale because I like playing high up the neck. However, the Rainsong Parlor nylon has brought me to an appreciation of a shorter scale. The 12 fret RS does not have my favored nut width and does not allow easy play beyond the 14th fret--but I believe that there is a critical advantage to a 12 fret instrument.

While I like long-scale cut-a-way guitars I believe that there is a difference in tonal quality and a significant difference in projection between such an instrument and a short scale guitar. And so, I have two guitars.

I think a 12 fret guitar with the Emerald nut width and neck thickness would be ideal. If the guitar also eliminated the heal I suspect that higher fret reach would not be a problem.