Kids these days! 😉
In all my years of full-time framebuilding, I never had much of a canti boss tool, despite most of the frames I made having cantis.
I had a piece of steel with two 6 mm holes in it. M6 screws though the piece of steel, threaded into the bosses. Miter the bosses so they match up with the blades or stays at the desired boss width. Measure the height (from the hub axle) with a tape measure.
Worthwhile addition: add a second piece of steel, with just one hole, in the center, tapped 1/4" x 20 to take a piece all-thread with a wingnut brazed to the end. It goes through the first piece I mentioned and into the second, pinches the stays or blades
between the 2 pieces of steel. Now the subassembly doesn't fall off if you bump it, hallelujah! And you can adjust the height of one without moving the other, and rotate it every which way for fluxing and brazing.
If this seems imprecise to you, all I can say is it was plenty precise enough, and that was back when cantis meant Mafac, with zero up-down adjustment range (other than the "nod" angle of the pad, but no up/down translation
independent of the angle) I figger my method was accurate to a mm, probably 0.5 mm with good lighting.
If you don't need your posts to be exactly 80
mm (or whatever) apart, i.e. if you're willing to compromise on that width to make your miters match up with the blades/stays, then slot one of the holes where the bosses screw on, to give you some width wiggle room. Or just make another top plate — it's
just a piece of steel from the scrap pile with 3 holes drilled in it, five minutes to make one if you're slow like me.
Or hold out for one that's all CNC milled, corner-rounded and orange anodized, with
engraved filigree like a Clickspring masterpiece. But the brakes won't feel any different out on the trail.
Mark B in Seattle