Hi Bob,
The crucial thing isn't the positioning of the rails, it is the curve on the bottom of the box that holds the router. The curve on the box is a 12" radius and the spacing of the rails allows the box to rotate over that radius with enough room for the fretboard under the jig. If the rails were further apart or closer together what would change is the relationship of how far away the fretboard would be from the router bit.
If I wanted to make a fretboard with a 10" radius I would just need to make a new box with that radius on the bottom. I could then use the same jig but would have to adjust the height of the platform under the fretboard so that it reaches the router bit.
Below are 2 pics that should show what I mean.
This is the 12" radius jig. (the one in the pics above)
This is a 10" modification. The only thing that has changed is the radius box that the router is attached to and one of the risers
Hope that helps.