I'm just about finished with my first guitar, and went with flat for about a 2.5" span under the fingerboard extension, and radius down to the border from there. Idea being to leave a flat surface for gluing, but still get some dome for humidity safety and proper geometry for the extension to lay down on the top with the neck set to the proper angle. Mostly going by Cumpiano&Natelson book, although he does a smooth radius all along the brace.
I draw a line along the brace at the arch height I want, and then hand plane until I get down to the line, being careful not to remove any material from the center. Aim for a spherical curve, but the exact curve doesn't matter so much as the total offset distance. Perimeter of the top is flat (jointed sides before bending, glued straight to the top with individual tentellones). 1/8" arch on the X, 1/16" on the upper transverse, which in my case came out to make the fingerboard extension float just a bit above the surface with the neck set for string height 1/2" or a tiny bit less above the top. Probably a good thing, as it will likely lay down flat under string tension, so should be good after flexing it down and gluing. Having it flat now might result in a "ski jump" fingerboard extension under tension, not so good
