Ah ,so the tuning is on the fans and not the bits in between . Fascinating stuff ."
My view of it is that tuning is a process of getting the fans, and the other bracing, into some sort of balance with the top plate itself, so that all the parts are working together efficiently in some sense. I start out by testing the top wood to determine the 'correct' thickness that will give me the stiffness of that I want for the top plate. The bracing is made over size, and trimmed to get the stiffness to work well with that top. Rather than testing each piece, and trying to select and size them separately before hand I work with the whole assembly. The top plate and the bracing work as a unit when they're together: there are makers who talk about tuning the bracing as if it were separate from the top, but it doesn't work like that.