[charles Tauber]

The question to ask is the geometry of the curvature of your top. If you provide the curvature in a curved dish, the geometry of the top, particularly the upper bout, is a portion of a sphere. If you provide the curvature by other means, the curvature of the top, particularly in the upper bout, is a cylinder. If it is cylindrical, it doesn't really matter what radius you use as the centreline of the top will lie on a single straight line. In that case, what matters is the angle of the neck relative to that centreline. If the angle of the neck tilts backwards - the head lower than the top - the fingerboard extension will need to be bent at the juncture of the body since it lies on a different plane that the centreline of the top. This is what happens in a neck reset - either the fingerboard extension is bent downwards, or a wedge is placed beneath it to maintain the plane of the fingerboard. (Classical guitars are often built with a neck angle in which the head is higher than the top and the fingerboard extension must be planed wedge-shaped - thinning from the neck juncture towards the soundhole - to allow the top of the fingerboard to line in a single plane.)

If you are using a spherical geometry, you will still have the same issue - the angle of the neck must lie on the same "plane" as the centreline of the top in the upper bout. Trevor Gore, in his books, provides a very detailed mathematical analysis of the geometry, and then simply says, "Oh, just flatten the upper bout with a sanding block", thus creating a flat plane, at an angle slightly larger than 90 degrees to the sides, to which the fingerboard surface of the neck must be coplanar. He also suggests using a shallower curve for the transverse braces of the upper bout, but that is another story.

In either case, look to the neck angle that you are using. On the finished instruments, what vertical distance do you have from the top to the bottom of the strings, at the bridge?

Another possible cause is that the neck is vertically misaligned from the top and sits proud. In this case, the fingerboard would need to be bent downwards to attach to the top and there would be a gap between the top and the bottom of the fingerboard adjacent to the neck joint.