As I noted in this post, I'm planning to have a cantilevered board at either end, to allow the trains to turn 180 degrees and trundle into the fiddle yard. These boards are designed to be 600mm wide (roughly 2 feet) - same width as the main boards.
I also want to use as large a radius of curve as possible to minimise loco running problems. And eventually I'd like to run my locos with the scale-size leading bogie wheel that GF and others now provide (although the smaller size wheel still look pretty good). So large radius curve will help.
The largest standard Settrack curve is Radius 3 (product number ST-17), which is 298.5mm (11 3/4in) radius. This 'should' fit my baseboard. But it doesn't, according to XTrkCad. I checked with the N Gauge Yahoo group, which confirmed this - radius measurements are to the centre of the track, not the outside. Why does no-one tell you this?!
Anyway, the simplest solution is to make the cantilever boards 50mm wider at the back (hidden side) of the layout. Minimum of difference in construction, and little (if any) additional cost.