The gizmos can actually be accurate if you happen to get a decent one. Where some of them lack is the box/shell. If its metal, put a knife edge rule on it in a few places & do the daylight test. It might not even hurt to dress it (if its metal) & this will help a lot with repeat-ability. But to do a quick check, just stick in on & off your mill vise or a 123 block at some random angle & it will give you a feel fro how much you can trust it. They are pretty handy actually. Some have a magnet & you can stick it to a parallel which gives you more contact area options.