Reply by MrRon

A Starrett square is a precision tool intended for machinist accuracy. It wasn’t intended for woodworking or framing a house, although it could be used for such. The garden variety square that you can buy at your home building supply store or Ace Hardware will not be as precise as the Starrett, nor does it need to be. For one thing, doing layouts with a pencil, no matter how sharp, will never get you a precise line. Even marking of metals with layout dye and needle pointed scribers are never 100% precise. Wood changes with temperature, humidity and internal stress and there isn’t much you can do to compensate for it. The goal is to use whatever tools you have and be as precise with what you have. Using a Starrett square to draw a line will still not be precise due to the pencil point which has thickness to it, so there’s no good reason not to use the generic brand square, as long as it is “reasonably” square. I have a Starrett square that I use only to check the squareness of generic squares and for metalworking. When not in use, it stays hidden away. It’s never used for general woodworking. The square I use for woodworking is a Stanley. It checks out pretty good by the Starrett; good enough for woodworking.
Just think about this for a moment; layout for a 45° miter,(2 possible errors); set the miter gauge on the saw to 45°, (1 possible error); the miter gauge has a sloppy fit in the slot, (1 possible error); make the cut with the saw, (1 possible error) due to blade or arbor runout. Add up the errors, (5) and your 45° miter is no more. I may go out on a limb and say “it is impossible to cut a perfect 45° miter”. If you do, it’s an accident and not intentional. So don’t beat yourself up trying to get perfection. We should always strive for it even if we know it can never be achieved. I’m certain if I was to lay out a 45° miter, one with the Starrett, and another with the Stanley, the final result would be the same. Just my 2¢ worth.