Thanks for the replies. I don't expect the stays to necessarily move the same amount, but they should be closer to equidistant from the centerline if I spread them together rather than using the Sheldon Brown 2x4 method. I use a 4-foot level between the dropouts and the head tube to measure the distance from the seat tube (the string method with a "better" string). That will be good enough for me. As for the clamp on the seat stays, they contact the stays below the bridge. Their faces are parallel to one another while the stays form more of a Λ shape. (Also, that pic was taken on my first try a few days ago, so may not reflect "refinements" in positioning.)

I'll try the 5mm increment thing. I'm happy nobody said, "That won't work. Your tubing is heat treated."