I always have to look up those numbers. Since the spacing was established, rims have grown 5mm wider, so you want to be at 80-85mm. I would also just put a wheel in and look at the way the brake shoe falls on the rim as a sanity check. Note that the Tektro brakes have really long slots. You want the brake shoe to be close to the pivot in order to get the most mechanical advantage. If you have them at the other end of the slot, they don't work that well. And also, Paul brakes will not fit because their slots are short.
My first canti fixture was a flat plate with a couple of holes in it. That was back before there were standard distances, I just figured out where they would hit the wheel most conveniently and brazed them there. Wonder how that matches up to the standard numbers.