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I've started cutting top braces and putting a radius on them. I'm using Kinkead's book as a guide, and he stresses that the braces should be rough cut taller than the finished size. Cumpiano says the same. Neither reference, and none of the others I have checked, is clear about why, and whether the x-braces are reduced to finished height before notching. If so, the notch would be cut to the halfway point of each brace. If not (and why not), then should I cut the notch so that it is centred on the finished height of the braces, not the centre of the rough cut height?

I do it a little differently as I pre shape the braces before I glue them up. In the methods you are using , you make your notch to fit the rough brace as they should be proper thickness IE 1/4 in 5/16 etc. Then when you carve them later you are more or less correcting for height and cross section.
When you shape the braces you keep the join in tact or at least flat to glue a cross member on the top of the bottom brace to aid in strengthening the open joint.

Presumably you'll contour the bottom of the braces to the radius you want the top to be (28' or whatever). After that's done, reference off the bottom surface of the braces to cut the notch where it ought to end up when the braces are cut down to finished size.